The permeability properties of polymeric membranes were tailored by blending polyurethane (PU) into a poly(dimethylsiloxane) (PDMS) matrix. The sorption and diffusion behaviors of methanol and toluene were investigated in the PU, PDMS, and blended membranes. The permeant solubilities in the membranes were governed by thermodynamic equilibria between the solvents and the membranes, and were related to the differences in their solubility parameters (delta). The incorporation of PU (delta = 21.73 J(1/2) cm(-3/2)) into PDMS (delta = 16.63 J(1/2) cm(-3/2)) resulted in a blend that was more compatible with methanol (delta = 29.2 J(1/2) cm(-3/2)), whereas the highest toluene (delta = 18.25 J(1/2) cm(-3/2)) solubility was observed in the 20/80 (wt%) PU/PDMS blend. Permeant diffusivity is associated with the polymer fractional free volume, which is dominated by the chain mobility of the polymeric substance. The free-volume characteristics of the membranes were determined using positron annihilation lifetime spectroscopy (PALS). The free-volume cavity size shrank and hole density was diminished at higher PU blending fractions compared to pure PDMS; the fractional free volume decreased monotonically with PU content due to the mobility confinement arising from the higher hard-segment level contribution of the PU. As a result, the methanol and toluene diffusion coefficients showed negative relationships with the PU content in the blended membranes. The dependence of the permeant diffusivity on fractional free volume could be fully described using free-volume theory. The results suggest that the transport properties of a membrane can be optimized by blending appropriate ratios of suitable polymer components. (C) 2010 Elsevier B.V. All rights reserved.