Dynamic (time-varying) competitive sorption, modulated through fluctuating relative humidity, is shown to significantly affect the diffusive flux of 1,2,4-trichlorobenzene through soil columns. The flux varied by a factor of up to 2 during a 24 hour cycle in the experimental system when subjected to humidity fluctuations of ¿40%. This effect was observed despite conditions in which sufficient water coverage would have been present to significantly depress volatile organic chemical (VOC sorption). Many vadose zone chemical transport models do not incorporate vapor-solid sorption, which is known to be important in low-moisture soils. The models that do incorporate vapor sorption do not account for dynamic variations of the partitioning behavior associated with variable moisture content. We present and validate a mathematical model for VOC transport through soils that accounts for dynamic nonlinear competitive sorption on volatile species transport. Simulations are presented which show that the effect of a thin dry zone at the soil-air interface can result in large variations in the instantaneous chemical release rate. This implies that for some conditions in the field, VOC sampling should be conducted over longer periods to avoid bias due to short-term spikes in the chemical release rate. ¿ 1999 American Geophysical Union