Abstract The sorption and diffusion of propane in a series of alternating block polymers of polydimethylsiloxane (PDMS) and poly(bisphenol-A carbonate) (BPAC) have been studied as a function of composition and block length in the temperature range 30 to 60°C. The Langmuir-type component to the sorption is reduced as the block length is decreased, reflecting to some extent the dependence of the glass transition temperature of the microdomains on block length. The permeabilities as a function of copolymer composition fall into distinct sets for the 20- and 40-block length materials. The temperature dependence of the permeability indicates a continuous phase rich in PDMS. The results are interpreted in terms of an earlier model in which microdomains of BPAC are dispersed in a continuous phse of PDMS containing some BPAC oligomer.