Simple deterministic models are useful tools for evaluating the relative environmental impact of chemicals. Analytical solutions that assume all soil parameters to be constant in time and depth have been used for screening purposes. In this study, two recently presented numerical models, the moving mean slope method for solving the unsteady water flow equations and the moving concentration slope method for solving the convective dispersion equation, were combined to calculate unsteady one-dimensional transport and volatilization of toxic organic compounds in soils. An expression for the combined gaseous and liquid flux (fluid flux) of chemicals was derived by integrating the fluid flux equation with respect to soil depth. The resulting model for evaluating the relative impact of chemicals at transient state (RIOCATS) is numerically simple, easy to program, and accurate if given criteria for time and depth increments are observed. Typical variations of soil parameters with depth and time, such as soil-water content, organic matter, and zone of microbial activity, resulted in a significant change in the ranking of volatilization of 18 different chemicals in a clay soil compared with a screening based on constant, average parameters. Therefore, the RIOCATS model appears to offer advantages for predicting the relative fate of toxic chemicals in soils.