A new free volume theory, which we named "shell-like free voltone" theory, is developed for penetrant diffusivity in polymers by introducing microscopic concept or molecular collisions. Shell-like free volume is defined as the ambient free space or the penetrant molecule; it is consistent with the notion of molecular collisions, which is the microscopic origin of molecular diffusion. The microscopic notion call give physical meaning to all the parameters in the theory, and the parameters can be evaluated using the only pure-component parameters: the experimental viscosity or the solvent, the viscoelasticity of the polymer, and the molecular surface area estimated from the chemical structure using the semiempirical quantum chemical calculation. The predictive ability of the shell-like free volume theory is good for self-diffusivities of molecules with shapes from spherical to chain-like in polymer solutions over wide ranges of temperature and concentration.