On’yomi 音読み refers to the Japanese approximation (or even transliteration in some cases) of the pronunciation of the Chinese characters at the time they were imported to Japan.

Cantonese is a language of the Chinese languages. There are several observable patterns in modern Cantonese phonology that correspond to On’yomi readings. It should be noted that Chinese characters were imported separately at different eras. There is no one general rule that governs the transliteration. Also, some characters may bear more than one On’yomi readings. Nevertheless, the below may serve for pedagogical purposes, or as mnemonics for Cantonese-speaking Japanese learners/ Japanese-speaking Cantonese learners.

This entry will be kept updated according to the author’s own Japanese study.

1. Since generally each Chinese character has only one syllable, an On’yomi reading will not have more than 2 moras.
2. The consonant ng- at the beginning becomes g- e.g. 牛 ngau4 > gyuu, 藝 ngai6 > gei, 岸 ngon6 > gan, 額 ngaak9 > gaku, 癌 ngaam4 > gan. An On’yomi starting with a “g” may suggest the character used to have a beginning “ng” consonant in Middle Chinese e.g. 義 yi6 > gi, 儀 yi4 > gi, 議 yi5 > gi.
3. Generally the consonant in modern Cantonese reading can be transliterated to the Japanese approximation  e.g. s- > s-/sh-, t- > t-/ch-, l- > r-, z- > z-/ch- etc. But h- in Cantonese may become k/g-, as in 氣(気) hei3 > ki, 海 hoi2 > kai, 學(学) hok9 > gaku, 校 hau6 > kou, 客 haak8 > kyaku; f- may become k-, as in 花 faa1 > ka, 火 fo2 > ka; m- may become b-, as in 母 mou3 > bo(u), 晚 man3 > ban, 物 mat9 > butsu. These represent sound change that happened in Cantonese after Japanese had adopted the On’yomi.
4. A short vowel in Modern Cantonese is generally also a short vowel in On’yomi, e.g. 主 zyu2 > ju, 雨 yu5 > u, 火 fo2 > ka, 書 syu1 > syo, 所 so2 > syo, 處(処) cyu5 > syo.
5. A short “o” may become “a” e.g. 阿 o1 > a, 我 ngo5 > ga, 火 fo2 > ka, 左 zo2 > sa, 可 ho2 > ka, 過 gwo3 > ka, 魔 mo1 > ma, 羅 lo4 > ra, 倭 wo1 > wa, 安 on1 > an. It has been argued that there has been an “a > o” vowel shift in Chinese.
6. -iu becomes -you e.g. 鳥 niu5 > chou, 尿 niu6 > nyou, 標 biu1 > hyou, 表 biu2 > hyou, 小 siu2 > syou, 超 ciu1 > chou, 朝 ziu1 > chou, 妖 yiu2 > you, 要 yiu3 > you.
7. -ng becomes a long vowel of either “u”, “e”, or “o” in On’yomi (there are no diphthongs in Modern Japanese. “ou” is just a long “o”, and “ei” is just a long “e”) e.g. 風 fung1 > huu, 空 hung1 > kuu, 港 gong2 > kou.
8. But if the On’yomi was adopted during or after the Tang Dynasty, -ng becomes -n e.g. 明 ming4 is read myou (Go’on) or mei (Kan’on) (both with a long vowel), but min for the meaning “Ming Dynasty” (this On’yomi was of course adopted during the time of the Ming Dynasty, which was after Tang).
9. Generally, -m becomes -n.
10. Unlike Mandarin, checked tones in Middle Chinese are preserved in both Modern Cantonese and Japanese On’yomi. -k in Cantonese will become -k(i/u) e.g. 式 sik7 > shiki, 夕 zik9 > seki, 國(国) kwok8 > koku, 學(学) hok9 > gaku; -t will become -chi/tsu e.g. 一 yat7 > ichi, 日 yat9 > nichi, 月 yuet9 > g(a/e)tsu, 切 cit8 > setsu.
11. -p in Cantonese, on the other hand, is irregular in On’yomi. It may become a long vowel in On’yomi e.g. 集/習 zaap9 > syuu, 十 sap9 > juu, 級 kap7 > kyuu, 葉 yip9 > you, 合 hap9 > gou, 塔 taap8 > tou, 甲 gaap8 > kou, 蠟(蝋) laap9 > rou, 協/脅 hip8 > kyou; or -tsu e.g. 立 laap9 > ritsu, 雜(雑) zaap9 > zatsu, 接 zip8 > setsu [This has something to do with a phonologically phenomenon called ハ行転呼]
12. Different pronunciations to differentiate meanings represented by the same character can also be seen in both, e.g. 重 is read zung6/chou in 貴重/尊重/珍重/慎重/莊(荘)重 (meaning precious/respect/important/prudent/respect); but read cung5/jyuu in 重複/重量 (meaning duplication/weight)