The Nukak language (Nukak Makú: Guaviare) is a language of uncertain classification, perhaps part of the macrofamily Puinave-Maku. It is very closely related to Kakwa.

There are six oral and six nasal vowels.

The vowel [u] becomes the labial semivowel [w] in several environments: in postnuclear position (when it appears immediately after the nuclear vowel of a morpheme), before another vowel, and at the beginning of the word or syllable. The semivowel [w] is devoiced (IPA symbol [ʍ]) if the tone rises and the following vowel is [i], [ĩ], [ɨ].

The vowel [i] becomes the palatal semivowel [j] in postnuclear position.

Nasalization in Nukak language is a prosodic property of the morpheme that affects all segments within each morpheme except voiceless stops. Each morpheme is either completely nasal or completely oral.

There are eleven consonant phonemes: /p/; /b/ (realized as [m] with a nasal vowel, otherwise as [mb] at the start of a word, and as [bm] at the end of a word); /t/; /d/ (realized as [n] with a nasal vowel, otherwise as [nd] at the start of a word, and as [dn] at the end of a word); /tʃ/ (realized as [ts] or [tʃ] in free variation); /ɟ/ (realized as [ɲ] in a nasal environment); /k/; /ɡ/ (realized as [ŋ] in a nasal environment); /ɺ/ (lateral sonorant, alternating with the approximant [ɹ], the tap [ɾ], and the lateral approximant [l]); /h/; /ʔ/ (the glottal stop).

The following table of consonant phonemes shows each phoneme followed by the corresponding letter in the Nukak alphabet, where different.

/ɺ/ is pronounced [t] when followed by /t/ and [d] when preceded by a voiced consonant. If /ɟ/ is preceded by [t] or [ʔ], it is pronounced voiceless [t͡ʃ]. With some infixes or prefixes, /ʔ/ is replaced by /n/ when it is followed by any vowel or /h/ or in nasal [n] suffixes.