A study of the effects of some chlorinated organic solvents on the physical and spectral properties of poly(ether ether ketone) (PEEK) has been carried out. Infra-red and Raman spectroscopy, as well as mass diffusion studies, were used to examine the consequences of exposing PEEK to carbon tetrachloride, methylene chloride, chloroform and tetrachloroethane. Certain solvents were found to cause swelling and induce crystallization in the polymer. The effects of these solvents on the Raman spectrum of PEEK allowed a fictive temperature of 250 degrees C to be assigned. Lewis acid-base theory was used to interpret the different swelling effects.