The diffusion of methylene chloride and tetrahydrofuran in amorphous poly(ether ether ketone) (PEEK) films was studied by the following method. A specimen was immersed in the solvent, the reaction was stopped by immersing the specimen in liquid nitrogen, the dried specimen was finally fractured into two pieces, one for observation under scanning electron microscope and the other for thermal analysis by differential scanning calorimetry. As the solvent was allowed more time to penetrate into the film, the fracture surfaces showed distinct solvent diffusion fronts that increased in thickness and the d.s.c. curves displayed crystallization exothermic peaks whose areas decreased, showing the solvent-induced crystallization (SINC) process. SING was then analysed in term of the diffusion process using the Zachmann and Konrad theory. The SINC process was found to be solvent-diffusion controlled.