Diffusion coefficients of water vapor in a polyimide homopolymer and poly(imide-siloxane) multi-block copolymers of different siloxane concentration were determined from gravimetric sorption experiments. Diffusion coefficients were of the order of magnitude of 10(-8) cm(2)/sec. Higher levels of siloxane incorporation caused a definite increase in the diffusion coefficient, indicating a decreased resistance to water ingression. The increase in diffusion was found to be influenced by siloxane block length and was interpreted in terms of free volume and morphology. The diffusion coefficient of the 10-weight-percent PDMS copolymer, however, was found to be the same within error as that of the polyimide. This and a previous surface study suggested that an increased surface water resistance may be achieved at low siloxane concentrations without greatly increasing the bulk diffusive properties to undesirable levels.