The kinetics of moisture uptake in a rigid gas permeable contact lens material Paraperm has been investigated. The free volume changes accompanied by the sorption of water have been monitored using Positron Lifetime Spectroscopy (PLS) technique. The positron results indicate the swelling in the initial stages of sorption which seems to be balanced by the rearrangement of chains in the presence of water molecules and filling of vacant free volume sites with water at the later stages of sorption. The sorption curve suggests that diffusion process in this polymer follows dual‐mode sorption model. Using this model we have been able to separate the Fickian controlled diffusion and relaxation controlled diffusion. Further, the positron results combined with sorption results indicate that Fujita's free volume theory seems to be not valid for the diffusion process in the present sample under study. The variation of permeability as a function of free volume suggests the amount of water in a contact lens is a measure of its permeability
