The free volume theory of diffusion for hard spheres, earlier developed by Cohen and Turnbull, is modified for simple van der Waals type liquids. The modified theory fits the self‐diffusion data for argon, krypton, and xenon fairly well and predicts glass transition for these liquids at approximately one‐third of their normal boiling points. It is found that a more accurate model for the free volume diffusion in liquids should include a redistribution energy for voids arising from the nonlinear behavior of the pair potential. The agreement found by Cohen and Turnbull between their hard sphere theory and the self‐diffusion data of some large organic molecules is interpreted as evidence that the simple Lennard‐Jones potential does not accurately describe their interaction with each other. This situation is reflected in the failure of these molecules to obey the same corresponding states relationship as the simple liquids of argon, krypton, and xenon.