High-temperature cation distributions in the iron-manganese mixed spinels have been investigated by in-situ Mossbauer spectroscopy. Mossbauer spectra have been measured at temperatures up to 1200-degrees-C. The spectra have been deconvoluted into contributions from octahedral and tetrahedral sites. Concentrations of the different charge states on the different sublattices have been determined using the respective isomer shifts. Results of this novel approach to cation distributions of complex spinels are reported for (Fe0.8Mn0.2)3O4.