In arid and semi-arid regions where net infiltration and soil moisture are relatively low, movement of volatile organic compounds (VOC) in the vadose zone may be dominated by vapor transport and adsorption mechanisms. The objective of this study is to develop the experimental methods necessary to determine the rate of isothermal adsorption and desorption, and the adsorption capacity of toluene (and other VOCs) onto different soils and minerals subject to a range of initial water contents and temperatures. The proposed method is a modification of the gravimetric method described by both Brooks and Grismer. A stainless steel chamber houses a load cell connected through a serial interface to a computer. After sealing the dry adsorbent (soil) in the chamber, the chamber air is purged using dewatered nitrogen gas before introduction of the adsorbate (VOC). The computer, through the interface, directs the load cell and records sample mass at preselected time intervals without additional supervision. The method is applied to measure rates of adsorption and desorption of toluene on bentonite. Toluene sorption rates are modeled using a second-order rate equation described by Grismer that yielded an initial rate parameter equivalent to that for water vapor adsorption despite the toluene having a much smaller adsorption capacity. The results illustrate the simplicity of the proposed method.