The important mechanism of interface layer formation in polymer composite materials is a sorption interaction between polymer fillers and binder components. The structure and properties of the interface layer will be dependent on the kinetics of the sorption. The kinetics of diffusion of some low molecular compounds including epoxies into aramide fibers are studied by the method of isothermic calorimetry. Discrepancy of the kinetics from Fickian was found. Theoretical consideration showed the cause of it could be the structural inhomogeneity of the sorbents. Experimental and theoretical data evidencing that the heterogeneous structure of the sorbent is the cause of the time dependence of the diffusion coefficient are presented in this paper.