The electrical conductivity of a range of ceramics of general formula Ca₂Mn_2 − xNb_xO_γ, with x and γ as variables, was measured using impedance spectroscopy. All materials were semiconducting but conductivity decreased with both increasing x and decreasing γ, both of which involved a gradual decrease in oxidation state of Mn from +4 in CaMnO₃ to +2 in Ca₂Mn_0.8Nb_1.2O_5.8. Possible factors that control the conductivity are discussed; electrons in e_g orbitals of Mn appear not to be involved directly; instead, conduction appears to depend either on the degree of t_2g–Op_π–t_2g orbital overlap or on σ d–p overlap, with donation of electrons from oxygen into the bonds involving e_g orbitals. In both cases, the conductivity is influenced mainly by Mn–O–Mn distances and unit cell dimensions. For compositions of intermediate oxygen content γ, it was difficult to obtain materials that were electrically homogeneous due to oxygen content gradients; thus, samples prepared by partial reduction in H₂ were more heavily reduced at the grain surfaces than in the grain interiors. Since the conductivity was very sensitive to γ, impedance spectroscopy provided a valuable method to investigate oxygen inhomogeneity. Two types of sample inhomogeneity were characterized: those with reduced surfaces and oxidized cores and vice versa, those with oxidized surfaces and reduced cores.