The in situ volatilization process was observed in two large soil columns, each 3.05 m in height and 0.915 m in diameter, packed with a fine sand, and constructed with a central well for production of air and vapors through a vacuum pump. After initially wetting the soil with water, a four-component hydrocarbon mixture was added to each column. In this paper, the observed data for hydrocarbon removal over fifteen months of operation are summarized. In addition, two models were formulated to model the process and interpret the experimental data. The first model, based on vapor-liquid equilibrium, predicted removal rates much higher than those observed. The second model, based on diffusion relationships, was used to calculate effective diffusion parameters to describe the observed data. Additional aspects of the study are discussed.