[5] It may be part of a Coast Oregon Penutian family together with Alsea and the Coosan languages, although the validity of this family is still controversial.

Proponents of the disputed Penutian phylum usually include Siuslaw as part of it, together with the other Coast Oregon Penutian languages.

[6] Published sources are by Leo J. Frachtenberg who collected data from a non-English-speaking native speaker of the Lower Umpqua dialect and her Alsean husband (who spoke it as a second language) during three months of fieldwork in 1911,[7][4][8] and by Dell Hymes who worked with four Siuslaw speakers in 1954.

[9] Further archived documentation consists of a 12-page vocabulary by James Owen Dorsey,[10] a wordlist of approximately 150 words taken by Melville Jacobs in 1935 in work with Lower Umpqua speaker Hank Johnson,[11] an audio recording of Siuslaw speaker Spencer Scott from 1941, hundreds of pages of notes from John Peabody Harrington in 1942 based on interviews with several native speakers,[12] and audio recordings of vocabulary by Morris Swadesh in 1953.

Cluster of stops/affricates + glottal stop are realized as ejective consonants: [pʼ, tʼ, tɬʼ, tsʼ, tʃʼ, kʼ].