Although the literature on Rundi agrees on 5 vowels, the number of consonants can vary anywhere from 19 to 26 consonants. The table below is compiled from a survey of academic acceptance of Rundi consonants.
Rundi is a tonal language. There are two essential tones in Rundi : high and low (or H and L). Since Rundi has phonemic distinction on vowel length, when a long vowel changes from a low tone to a high tone it is marked as a rising tone. When a long vowel changes from a high tone to a low tone, it is marked as a falling tone.
Rundi is often used in phonology to illustrate examples of Meeussen's rule In addition, it has been proposed that tones can shift by a metrical or rhythmic structure. Some authors have expanded these more complex features of the tonal system noting that such properties are highly unusual for a tone system.
Syllable structure in Rundi is considered to be CV, that is having no clusters, no coda consonants, and no complex vowel nuclei. It has been proposed that sequences that are CVV in the surface realization are actually CV in the underlying deep structure, with the consonant coalescing with the first vowel.
Rundi has been shown to have properties of consonant harmony particularly when it comes to sibilants. Meeussen described this harmony in his essay and it is investigated further by others. One example of this harmony is triggered by /ʃ/ and /ʒ/ and targets the set of /s/ and /z/ in preceding adjacent stem syllables.