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Solubility and diffusion coefficients for C02 in polystyrene-polybutadiene (SB) block copolymer films were measured in a pressure decay sorption apparatus at 1 atm over a temperature range of 20-90 °C. The morphology of the block copolymer specimens consisted of alternating S and B lamellae of ca. 100-A thickness with excellent long-range orientation perpendicular to the surface of the 0.02-0.05-cm-thick films. Diffusion behavior in the copolymer was simulated by using homopolybutadiene and homopolystyrene data along with a finite-difference model based on the well-ordered morphology. Comparisons of the model predictions with the measured values of the effective diffusion coefficient for C02 in the copolymer revealed that the diffusion coefficient of C02 in the polybutadiene region of the copolymer was lower than in homopolybutadiene by a factor of 1.9 at 20 °C. This difference disappeared at the glass transition temperature of the polystyrene in SB. This temperature-dependent reduction of C02 diffusion in the polybutadiene regions of the block copolymer results from restrictions on chain motions of the polybutadiene blocks arising from their connection to the polystyrene blocks.