The atlas-axis complexes of the better-known genera of the Permo-Pennsylvanian tetrapod suborder Diadectomorpha are described and compared with those of other late Palaeozoic tetrapods. One new synapomorphy of the Diadectomorpha is identified: a large, anteriorly directed, midventral, projection of the axial intercentrum that articulates with a midventral furrow on the posterior margin of the atlantal intercentrum. Within the Diadectomorpha diadectids are apomorphic in possessing a large, anteriorly tapering axial neural spine, that accounts for approximately 40% of the total height of the axis; Limnoscelis is apomorphic in having divided parapophyseal facets of the atlantal intercentrum and anteriorly directed ventral processes of the atlantal intercentrum. Relative to the atlas-axis complex in the more primitive amphibian suborder Seymouriamorpha, that of diadectomorphs and other basal amniotes share two derived features: the atlantal pleurocentrum is fused to the dorsal surface of the axial intercentrum, except in Tseajaia where the elements are not fused but are positioned similarly; and the axial pleurocentrum and neural arch are fused in all observable stages of ontogeny. Reinterpretation of the atlantal neural arch in Seymouria indicates that it does not possess neural spines, a feature it shares with the Diadectomorpha and basal amniotes.