The microwave susceptibility of NiFe2O4 and Co2Z was measured between 100 and 10000 MHz. The samples were either samples with different porosities or composites with different volume fractions, in the case of NiFe2O4, or composites with different granulometry, in the case of Co2Z. The measured susceptibility undergoes substantial changes at the crossover from a ceramic state to a composite of isolated particles and for different granulometries of Co2Z in the range 10 to 600 µm. We show that a model in which the susceptibility is uniform inside the particles cannot account for these results. We suggest that in order to compute the susceptibility of composites, a microscopic approach is required
