Vapor phase diffusion is an important transport process in the unsaturated zone affecting evaporation of volatile organic compounds (VOCs) from pure and multicomponent organic liquids. To evaluate some widely used empirical relationships for the estimation of effective diffusion coefficients in the unsaturated zone and to assess the validity of Raoult's Law during aging of organic mixtures, two series of laboratory-scale column experiments were performed using pure toluene, pure methyl tert-butyl ether (MTBE), and two multicomponent “kerosene-type” liquids containing four to seven compounds. The analytical one-dimensional solution of Fick's Second Law described the diffusion process of pure compounds very well in two sands with different water contents. The effective diffusion coefficients obtained correspond well to a recently published empirical relationship (Moldrup et al., 2000); the capacity factors fitted indicate equilibrium partitioning of the solute between gas phase and water. A one-dimensional numerical model based on the combination of Fick's Second Law and Raoult's Law was used to predict the volatilization and the diffusion process from multicomponent organic liquids. Both the vapor phase diffusion process of the VOCs and the aging of the organic mixtures were predicted very well solely on the basis of effective diffusion coefficients estimated from the empirical relationship and assuming an ideal mixture (e.g., an activity coefficient of 1 in Raoult's Law).