Information about the distribution of cations between tetrahedral (A) and octahedral (B) sites in the spinel structure of NiMn2O4 and some substituted spinel phases are derived from thermopower measurements in connection with some ion distribution models. The cubic spinel NiMn2O4 is characterized by a high degree of inversion coupled with disproportionation of Mn(III) with formation of Mn(II) on tetrahedral and Mn(IV) on octahedral sites. Spinels of the series LizNiMn2-zO4 show increased concentrations of Mn(IV) on the B-sites and the Mn(II) content on the A-sites is reduced FeNiMnO4 is dominated by partial Fe octahedral site occupation connected with Mn disproportionation.