Sorption and permeation of three chloromethane vapors in poly(dimethylsiloxane) (PDMS) membrane have been investigated. Sorption increases with increasing number of chlorine atoms or molar volume, as it would be predicted from simple solubility perameter considerations. Permeability, however shows a distinct maximum for chloroform. The maximum in pormeability for chloroform can be qualitatively explained by solution-diffusion mechanism.