Abstract The preparation of Ni 6MnO 8 and NiMnO 3 - δ (δ ≈ 0.02) by thermal decomposition of oxalate mixed crystals of appropriate composition and the defect structure of the latter compound are reported. Magnetic measurements indicate localized d electron states. For temperatures above 75 °C the agreement between the thermal activation energy deduced from thermopower and conductivity measurements is discussed in terms of an intrinsic mechanism. The activation energy of the thermopower is found to be about twice the value expected for polaron hopping conduction. This and the occurrence of thermally activated thermopower strongly indicates the formation of extended states as a result of thermal charge carrier excitation.