This work concerns the study of the effect of adding microgranules containing a phase change material (PCM) on the thermal diffusivity value of the resulting composite structure. Two commercially available epoxy resins were applied as the composite matrix material: pure epoxy resin and epoxy resin with a filler in the form of metal powder. The dispersed phase was the BASF Micronal DS5038 X microgranulate, i.e. a bed of polymer spherical shells containing PCM filling. The tests were carried out using the modified Ångström method in a symmetrical bilateral harmonic excitation of the temperature of outer sides of assembled two tested disc-shaped samples. The temperature range of measurements covered the interval from 0°C to 35°C, i.e. most of the typical working range of microgranules. In selected measurement cases, the tests were extended to temperatures from ‒10°C to 75°C. The results obtained for the four structures studied were compared and analyzed to illustrate the quantitative effects of structure modification and to document the qualitative effects of the observed phase changes that occurred both on heating and cooling.