It includes all the Niger-Congo languages and subfamilies except the families of the erstwhile Atlantic and Kordofanian branches, Mande, Dogon, and Ijo.

In the infobox at the right, the languages which appear to be the most divergent (including the dubious Senufo and Kru, which may not be Volta–Congo at all) are placed at the top, whereas those closer to the core (the similar "Benue–Kwa" branches of Kwa, Volta–Niger and Benue–Congo) are near the bottom.

Comparative linguistic research by John M. Stewart in the sixties and seventies helped establish the genetic unity of Volta–Congo and shed light on its internal structure, but the results remain tentative.

This had been suggested before by Bennet (1983 as cited in Williamson and Blench 2000:17) in the case of the Gur and Adamawa–Ubangi languages, which apart from Ubangian are now linked together as Savannas.

Casali (1995) defends the hypothesis that Proto-Volta–Congo had a nine- or ten-vowel system employing vowel harmony and that this set has been reduced to a seven vowel-system in many Volta–Congo languages.