diff --git a/nisaba/scripts/brahmic/data/Beng/mni/vowel.textproto b/nisaba/scripts/brahmic/data/Beng/mni/vowel.textproto
index 903a4806..a2850691 100644
--- a/nisaba/scripts/brahmic/data/Beng/mni/vowel.textproto
+++ b/nisaba/scripts/brahmic/data/Beng/mni/vowel.textproto
@@ -36,15 +36,15 @@ item { uname: "E" raw: "এ" to_raw: "ē" }
 item { uname: "O" raw: "ও" to_raw: "ō" }
 
 # Atiya + Cheinap /ɐi/.
-item { uname: "AI"        raw: "ঐ"  to_raw: "ai" }
-item { uname: ["A",  "I"] raw: "অই" to_raw: "ai" }
+item { uname: "AI"        raw: "ঐ"  to_raw: "əi" }
+item { uname: ["A",  "I"] raw: "অই" to_raw: "əi" }
 
 # Atiya + Sounap /ɐu/.
-item { uname: "AU"        raw: "ঔ"  to_raw: "au" }
-item { uname: ["A", "U"]  raw: "অউ" to_raw: "au" }
+item { uname: "AU"        raw: "ঔ"  to_raw: "əu" }
+item { uname: ["A", "U"]  raw: "অউ" to_raw: "əu" }
 
 # Atiya + Nung /ɐŋ/.
-item { uname: ["A", "ANUSVARA"] raw: "অং" to_raw: "aṁ" }
+item { uname: ["A", "ANUSVARA"] raw: "অং" to_raw: "əṁ" }
 
 # TODO: Double-check.
 item { uname: "VOCALIC R" raw: "ঋ" to_raw: "ri" }
diff --git a/nisaba/scripts/brahmic/data/Beng/mni/vowel_sign.textproto b/nisaba/scripts/brahmic/data/Beng/mni/vowel_sign.textproto
index 59396388..3ff0b909 100644
--- a/nisaba/scripts/brahmic/data/Beng/mni/vowel_sign.textproto
+++ b/nisaba/scripts/brahmic/data/Beng/mni/vowel_sign.textproto
@@ -35,10 +35,10 @@ item { uname: "SIGN E" to_raw: "ē"  }
 item { uname: "SIGN O"  to_raw: "ō" }
 
 # Sounap /ɐu/.
-item { uname: "SIGN AU" to_raw: "au" }
+item { uname: "SIGN AU" to_raw: "əu" }
 
 # Cheinap /ɐi/.
-item { uname: "SIGN AI" to_raw: "ai" }
+item { uname: "SIGN AI" to_raw: "əi" }
 
 # TODO: Double-check.
 item { uname: "SIGN VOCALIC R" to_raw: "ri" }
diff --git a/nisaba/scripts/brahmic/data/Mtei/vowel.textproto b/nisaba/scripts/brahmic/data/Mtei/vowel.textproto
index 30f28e6e..f365f03a 100644
--- a/nisaba/scripts/brahmic/data/Mtei/vowel.textproto
+++ b/nisaba/scripts/brahmic/data/Mtei/vowel.textproto
@@ -28,6 +28,11 @@ item { uname: ["ATIYA", "SIGN INAP"]    raw: "ꯑꯤ" to_raw: "ī" }
 item { uname: ["ATIYA", "SIGN UNAP"]    raw: "ꯑꯨ"  to_raw: "ū" }
 item { uname: ["ATIYA", "SIGN YENAP"]   raw: "ꯑꯦ" to_raw: "ē" }
 item { uname: ["ATIYA", "SIGN ONAP"]    raw: "ꯑꯣ" to_raw: "ō" }
-item { uname: ["ATIYA", "SIGN CHEINAP"] raw: "ꯑꯩ" to_raw: "ai" }
-item { uname: ["ATIYA", "SIGN SOUNAP"]  raw: "ꯑꯧ" to_raw: "au" }
-item { uname: ["ATIYA", "SIGN NUNG"]    raw: "ꯑꯪ" to_raw: "aṁ" }
+
+# The "real", more accurate output representation of diphthongs below
+# that matches the above output notation, is `ai`, `au` and `aṁ`. But
+# we map them here to unique output symbols to avoid non-determinism
+# by replacing `a` with `ə`.
+item { uname: ["ATIYA", "SIGN CHEINAP"] raw: "ꯑꯩ" to_raw: "əi" }
+item { uname: ["ATIYA", "SIGN SOUNAP"]  raw: "ꯑꯧ" to_raw: "əu" }
+item { uname: ["ATIYA", "SIGN NUNG"]    raw: "ꯑꯪ" to_raw: "əṁ" }
diff --git a/nisaba/scripts/brahmic/data/Mtei/vowel_sign.textproto b/nisaba/scripts/brahmic/data/Mtei/vowel_sign.textproto
index 3afe5813..0d19442b 100644
--- a/nisaba/scripts/brahmic/data/Mtei/vowel_sign.textproto
+++ b/nisaba/scripts/brahmic/data/Mtei/vowel_sign.textproto
@@ -40,11 +40,14 @@ item {
   uname: "SIGN ONAP" raw: "ꯣ"
   to_raw: "ō"
 }
+
+# Please see the corresponding note on output mapping symbols for diphthongs
+# in `vowel.textproto`.
 item {
   uname: "SIGN CHEINAP" raw: "ꯩ"
-  to_raw: "ai"
+  to_raw: "əi"
 }
 item {
   uname: "SIGN SOUNAP" raw: "ꯧ"
-  to_raw: "au"
+  to_raw: "əu"
 }
diff --git a/nisaba/scripts/brahmic/testdata/iso.tsv b/nisaba/scripts/brahmic/testdata/iso.tsv
index 9191f19b..15e62a4c 100644
--- a/nisaba/scripts/brahmic/testdata/iso.tsv
+++ b/nisaba/scripts/brahmic/testdata/iso.tsv
@@ -122,19 +122,19 @@ FROM_BENG	নিঃহৗঁত	niḥhăm̐ta
 # ------------------------------------------------------------------------------
 # Meetei Mayek (Manipuri):
 # ------------------------------------------------------------------------------
-FROM_MTEI	ꯑꯩꯅ	aina
+FROM_MTEI	ꯑꯩꯅ	əina
 FROM_MTEI	ꯀꯣꯛ	kōk'
 FROM_MTEI	ꯁꯝ	sam'
 FROM_MTEI	ꯑꯇꯤꯌꯥ	atīyā
 FROM_MTEI	ꯆꯤꯟ	cʰīn'
 FROM_MTEI	ꯒ꯭ꯂꯥꯁ	glāsa
 FROM_MTEI	ꯀꯐꯣꯢ	kapʰōi'
-FROM_MTEI	ꯀꯨꯁꯨꯝꯂꯩ	kūsūm'lai
+FROM_MTEI	ꯀꯨꯁꯨꯝꯂꯩ	kūsūm'ləi
 FROM_MTEI	ꯀꯦꯇꯨꯀꯤ	kētūkī
 FROM_MTEI	ꯀꯦꯇꯦꯀꯤ	kētēkī
 FROM_MTEI	ꯀꯣꯝꯂꯥ	kōm'lā
 FROM_MTEI	ꯁꯤꯡ	sīṅ'
-FROM_MTEI	ꯂꯩꯁꯥꯕꯤ	laisābī
+FROM_MTEI	ꯂꯩꯁꯥꯕꯤ	ləisābī
 FROM_MTEI	ꯃꯌꯥꯡꯇꯣꯟ	mayāṅ'tōn'
 FROM_MTEI	ꯃꯦꯊꯤ	mētʰī
 FROM_MTEI	ꯄꯨꯗꯤꯅꯥ	pūdīnā
@@ -142,7 +142,7 @@ FROM_MTEI	ꯅꯨꯁꯤꯍꯤꯗꯥꯛ	nūsīhīdāk'
 FROM_MTEI	ꯆꯝꯄ꯭ꯔꯥ	cʰam'prā
 FROM_MTEI	ꯇꯨꯜꯁꯤꯄꯝꯕꯤ	tūl'sīpam'bī
 FROM_MTEI	ꯌꯥꯢꯉꯪ	yāi'ṅaṁ
-FROM_MTEI	ꯍꯩꯅꯧ	hainau
+FROM_MTEI	ꯍꯩꯅꯧ	həinəu
 FROM_MTEI	ꯎꯁꯤꯡꯁꯥ	usīṅ'sā
 FROM_MTEI	ꯑꯗꯨꯔꯒꯨꯂꯥꯕ	adūragūlāba
 FROM_MTEI	ꯒꯨꯂꯥꯕ	gūlāba
@@ -150,19 +150,19 @@ FROM_MTEI	ꯖꯥꯢꯐꯜ	jāi'pʰal'
 FROM_MTEI	ꯀꯥꯡ	kāṅ'
 FROM_MTEI	ꯀꯥ꯬ꯉ	kā^ṅa
 FROM_MTEI	ꯂꯦꯡꯖꯨꯝ	lēṅ'jūm'
-TO_MTEI	aina	ꯑꯩꯅ
+TO_MTEI	əina	ꯑꯩꯅ
 TO_MTEI	kōk'	ꯀꯣꯛ
 TO_MTEI	sam'	ꯁꯝ
 TO_MTEI	atīyā	ꯑꯇꯤꯌꯥ
 TO_MTEI	cʰīn'	ꯆꯤꯟ
 TO_MTEI	glāsa	ꯒ꯭ꯂꯥꯁ
 TO_MTEI	kapʰōi'	ꯀꯐꯣꯢ
-TO_MTEI	kūsūm'lai	ꯀꯨꯁꯨꯝꯂꯩ
+TO_MTEI	kūsūm'ləi	ꯀꯨꯁꯨꯝꯂꯩ
 TO_MTEI	kētūkī	ꯀꯦꯇꯨꯀꯤ
 TO_MTEI	kētēkī	ꯀꯦꯇꯦꯀꯤ
 TO_MTEI	kōm'lā	ꯀꯣꯝꯂꯥ
 TO_MTEI	sīṅ'	ꯁꯤꯡ
-TO_MTEI	laisābī	ꯂꯩꯁꯥꯕꯤ
+TO_MTEI	ləisābī	ꯂꯩꯁꯥꯕꯤ
 TO_MTEI	mayāṅ'tōn'	ꯃꯌꯥꯡꯇꯣꯟ
 TO_MTEI	mētʰī	ꯃꯦꯊꯤ
 TO_MTEI	pūdīnā	ꯄꯨꯗꯤꯅꯥ
@@ -170,7 +170,7 @@ TO_MTEI	nūsīhīdāk'	ꯅꯨꯁꯤꯍꯤꯗꯥꯛ
 TO_MTEI	cʰam'prā	ꯆꯝꯄ꯭ꯔꯥ
 TO_MTEI	tūl'sīpam'bī	ꯇꯨꯜꯁꯤꯄꯝꯕꯤ
 TO_MTEI	yāi'ṅaṁ	ꯌꯥꯢꯉꯪ
-TO_MTEI	hainau	ꯍꯩꯅꯧ
+TO_MTEI	həinəu	ꯍꯩꯅꯧ
 TO_MTEI	usīṅ'sā	ꯎꯁꯤꯡꯁꯥ
 TO_MTEI	adūragūlāba	ꯑꯗꯨꯔꯒꯨꯂꯥꯕ
 TO_MTEI	gūlāba	ꯒꯨꯂꯥꯕ
@@ -178,6 +178,12 @@ TO_MTEI	jāi'pʰal'	ꯖꯥꯢꯐꯜ
 TO_MTEI	lēṅ'jūm'	ꯂꯦꯡꯖꯨꯝ
 TO_MTEI	kā^ṅa	ꯀꯥ꯬ꯉ
 
+# Word-initial diphthongs.
+FROM_MTEI	ꯑꯧꯇꯗꯣꯔꯒꯤ	əutadōragī
+FROM_MTEI	ꯑꯧꯠ	əut'
+TO_MTEI	əutadōragī	ꯑꯧꯇꯗꯣꯔꯒꯤ
+TO_MTEI	əut'	ꯑꯧꯠ
+
 # ------------------------------------------------------------------------------
 # Baybayin (Tagalog).
 # ------------------------------------------------------------------------------