Proportions of reversible and irreversible magnetization processes in the overall magnetization processwere studied in a complex view of magnetic, thermal and electrical properties of iron–phenolphormaldehyderesin composites. They were determined experimentally at different values of magnetic inductionalong the initial curve. The results of total, differential, reversible and irreversible permeability measurementas well as the analysis of DC energy losses revealed the same tendencies: The numbers of movabledomain walls (determining the extent of reversible processes) depend on the magnetic particle size andthe resin content through the demagnetizing fields produced by the particle surfaces, lowering the interactionbetween particles. Thermal diffusivity was compared with Hashin–Shtrikman model indicatinggood insulation of particles.