A model for evaporation of chemical warfare agents on the ground has been developed. The process of evaporation is described in three steps: (1) the immediate drop enlargement due to impact momentum is modeled using an empirical correlation from technical literature; (2) further enlargement caused by capillary spreading upon the surface and the simultaneous sorption into the substrate, modeled in three dimensions; (3) subsequent drying and redistribution of the sorbed material is described as a one-dimensional (vertical) process. The formulation of the flux in the soil takes into account vapour, liquid, solute, and adsorbed phases. The evaporation from the surface is determined by the vapour concentration at the surface and the conditions in the atmospheric viscous sub-layer close to the droplet spots on the surface. Model results agree with the limited experimental data found in the literature. The model shows a very rapid sorption and redistribution of chemical warfare droplets on sand. This effect gives a rapid decrease of the evaporation, except for a shorter initial period. However, a small residual evaporation exists for a rather long time from liquid, which has penetrated down into the soil. © 1998 Elsevier Science B.V.