The diffusion of radioactively tagged n-hexadecane in trace amounts has been studied in 22 random styrene–butadiene (SBR) copolymers with different styrene contents and butadiene microstructures; in several SBR block copolymers with different average block lengths (also diffusion of tagged 1,1-diphenyl ethane); in a triblock SBR copolymer cast from different solvents and also molded at elevated temperature; and in cis-polybutadiene filled to different extents with carbon black, calcium carbonate, and microglass spheres. The diffusion coefficient in random SBR copolymers decreased with increasing content of styrene and/or vinyl configuration and could be correlated with fractional free volume on the basis of linear additivity of the cis, trans, vinyl, and styrene moieties. In SBR block copolymers, the diffusion coefficient increased with increasing average block sequence length. For the triblock copolymer, the diffusion coefficient was approximately the same for samples molded or cast from solvents which are good for polybutadiene, but was far smaller for a sample cast from ethyl acetate, in which the polystyrene domains are probably lamellar. The effect of fillers on diffusion in cis-polybutadiene was compared with calculations on the basis of several theoretical models.