The relationship between polymer side-chain length and the hole free volume that is effective for solvent diffusion was investigated for polyacrylates and polymethaerylates on the basis of free-volume theory. Measurements of a polymer's viscoelasticity and solvent diffusivity provided experimental evidence for polymer segment mobility, and the results indicated that hole free volume in a linear polymer increases with hydrocarbon side-chain length. Because the molecular mechanisms of polymer viscoelasticity and diffusivity are identical, the free-volume parameters obtained for polyacrylates and polymethacrylates by measuring the polymer viscoelastic-temperature dependence can reliably be used in predicting the solvent diffusion coefficient. (C) 2003 Wiley Periodicals, Inc.