Toposa, like many Nilotic languages, has vowel harmony with two sets of vowels: a set with the tongue root advanced (+ATR) and a −ATR set. +ATR is marked. The vowel /a/ is neutral with respect to vowel harmony.
All nine vowels also occur as devoiced, contrasting with their voiced counterparts. These voiceless vowels occur primarily in prepause contexts. Some Toposa morphemes consist only of a high voiceless vowel; the functional load appears to be much greater with the high vowels than with the lower.
Toposa has tone, which is grammatical rather than lexical. Tone is used to mark case in nouns and tense in verbs.
