The decomposition of freeze-dried mixed carboxylates of manganese and iron was investigated by means of DTA, TG, mass spectroscopy and X-ray powder diffractometry. The three main steps of decomposition are characterized as release of (a) H2O, (b) carboxylic acid and CO2/CO, and (c) the corresponding carbonyl compound and CO2. In particular, the course of process (b) strongly depends on the stability of the metal-carboxylate link in the three investigated carboxylates. Well-crystallized single-phase manganese ferrites can be obtained on decomposition of formates of appropriate composition and thermal treatment of decomposition products at 600 degrees C while maintaining a p(O-2) within the coexistence field of manganese ferrite.