The Ijoid, or perhaps just Ijaw, languages form a divergent branch of the Niger–Congo family and are noted for their subject–object–verb basic word order, which is otherwise an unusual feature in Niger–Congo, shared only by such distant branches as Mande and Dogon. Like Mande and Dogon, Ijoid lacks even traces of the noun class system considered characteristic of Niger–Congo, and so may have split early from that family. Linguist Gerrit Dimmendaal doubts its inclusion in Niger–Congo altogether and considers the Ijaw/Ijoid languages to be an independent family.
- Jenewari, Charles E. W. (1989) 'Ijoid'. In Bendor-Samuel, John and Hartell, Rhonda L. (eds.), The Niger–Congo languages: A classification and description of Africa's largest language family, 105-118. Lanham, MD: University Press of America.
- Williamson, Kay. 1969. 'Igbo' and 'Ịjọ', chapters 7 and 8 in: Twelve Nigerian Languages, ed. by E. Dunstan. Longmans.
- Williamson, Kay. 1971. The Benue–Congo languages and Ịjọ. In: Current Trends in Linguistics, Vol. 7, series ed. by T. A. Sebeok, 245-306.
- Williamson, Kay. 1988. Linguistic evidence for the prehistory of the Niger Delta. In: The Prehistory of the Niger Delta, ed. by E.J. Alagoa and others. Hamburg: Helmut Buske Verlag.
- Williamson, Kay. 1998. Defaka revisited. The multi-disciplinary approach to African history, edited by Nkparom C. Ejituwu, Chapter 9, 151-183. Port Harcourt: University of Port Harcourt Press.
- Williamson, Kay. 2004. The language situation in the Niger Delta. Chapter 2 in: The development of Ịzọn language, edited by Martha L. Akpana, 9-13.
- Williamson, Kay & Blench, Roger (2000) 'Niger–Congo', in Heine, Bernd and Nurse, Derek (eds) African Languages: An Introduction. Cambridge: Cambridge University press, pp. 11–42.
- Nordhoff, Sebastian; Hammarström, Harald; Forkel, Robert; Haspelmath, Martin, eds. (2013). "Ijoid". Glottolog. Leipzig: Max Planck Institute for Evolutionary Anthropology.
- Roger Blench, Niger-Congo: an alternative view
- Dimmendaal, Gerrit Jan (2011-01-01). Historical Linguistics and the Comparative Study of African Languages. John Benjamins Publishing. ISBN 9027211787.