Poly-ether-ether-ketone (PEEK) is a thermoplastic tough polymer used as a matrix for advanced composite materials in aeronautic applications. The investigation of its resistance to humid environment and to exposure to organic and chlorinated solvents is extremely important. A series of integral sorption experiments were performed on PEEK with three different kinds of penetrant, namely water, methylene chloride and methylene chloride-n-heptane mixtures. Coupled diffusion and relaxation phenomena were observed to occur in most of the adopted experimental conditions. Moreover, for the case of methylene chloride sorption at medium and high activities a solvent-induced crystallization phenomenon was also detected. The sorption behavior was found to range from ideal Fickian diffusion (where the molecular mobility is not affected by the penetrant concentration) to so-called anomalous diffusion. In the latter case the penetrant mobility is a complex function of penetrant concentration, time and temperature. An overview of theoretical models reported in the literature to describe such phenomena is also presented with the aim of interpreting the experimental results obtained.