A thermodynamic calculation is performed for processes occurring in the Fe-O-C-H system during decomposition-reduction of iron oxalate in a gaseous medium formed in the thermal destruction of high-molecular hydrocarbons. This makes it possible to choose optimal temperature conditions and ratios of components for obtaining magnetic iron-based powders for medicine with required phase composition. It is shown that, under the actual kinetic experimental conditions, the products of decomposition-reduction of iron oxalate in a complex gaseous medium are thermodynamically favored phases.