In 1991, linguist Geoffrey Kimball estimated the number of speakers of the language at around 400 people, of whom approximately 350 live in Louisiana.

For example, vowel length distinguishes meaning for palana "bean" and palaana "plate", as well as choba "big" and chooba "horse".

In Koasati, the end of a phrase is basically marked by either deletion of the final unaccented vowel or the nasalization of the final vowel when deleting it would eliminate phonological information relevant to the phrase's meaning.

For example, the final vowel in hopoonilaho̱ "he/she will cook it" is nasalized instead of deleted, and therefore is distinguished from the more emphatic hopoonilaha̱, where the irrealis future suffix -laha- indicates that the action will certainly occur, whereas the irrealis future suffix -laho- does not provide such certainty.

However, the website for the Koasati Language Project explicitly states that these consonants are never aspirated.

[8] The [ɦ] occurs most often before [t͡ʃ], as in [haɦt͡ʃí] "river", spelled ⟨hahchi⟩ in the official orthography, thereby distinguished from ⟨hachi⟩ "tail".

Koasati is a polysynthetic language with fairly extensive verbal prefixing and suffixing.

The am-set generally identifies alienable possession and a relatively small set of kinship terms and body parts, while the ca-set identifies inalienable possession and most kinship terms and body parts.

Kimball provides the following example as a possibility: o-st-oh-im-ilá-꞉ci-halpí꞉sa-laho-꞉li-má꞉mi-mpa-y-ongo:&-INSTR-DISTR-3.DAT-arrive-PL-ABIL-IRR-DEDUC-HSY-CNSQ-SW:FOCim-ca-yím-ko-˛3.STAT.OBJ-1SG.STAT-believe-3.NEG(1A)-PHR:TERMo-st-oh-im-ilá-꞉ci-halpí꞉sa-laho-꞉li-má꞉mi-mpa-y-on im-ca-yím-ko-˛go:&-INSTR-DISTR-3.DAT-arrive-PL-ABIL-IRR-DEDUC-HSY-CNSQ-SW:FOC 3.STAT.OBJ-1SG.STAT-believe-3.NEG(1A)-PHR:TERM"They say that they all might be able to go and bring it to him, but on the contrary, I do not believe them."

Unknown glossing abbreviation(s) (help);In the first word, all units that follow the root -ilá- "arrive" are suffixes filling the various position classes.

Muskogean languages such as Koasati have a three-way number distinction in their verbs, with singular, dual, and plural forms.

In Koasati, the h-grade is used to create a polite imperative as well as to indicate a sequence of actions.

For example, the verb óntin "to come" (singular, dual subject) changes to the imperative ónhtįh "Come on over!"

with the h-grade (in addition to the delayed imperative suffix marked by the vowel nasalization and final h).

To form the n-grade, the vowel of the verb root's penultimate syllable is nasalized and accented.

Koasati sentences generally follow a subject, object, verb (SOV) pattern.

Because Koasati uses the nominative case, these orders are not rigid - elements can be moved within the sentence for emphasis.