Using the assumption that adsorption as a function of time may be expressed by an exponential equation, viz. M = g + het/, it is possible to obtain the amount of water vapour adsorbed by a composite soil without waiting for equilibrium, which usually takes a long time. Given the experimental data for the amounts adsorbed versus time, one can determine g, h and , together with the amounts adsorbed at equilibrium by extrapolating the above equation to t . It is also possible to calculate the error trends in these parameters as a function of time by comparing the values at time t with those obtained for the longest experimental time. The error trends of the equation with time arise from the comparison of the experimental values with those predicted by the exponential equation. We have discovered that although different lengths of time are necessary for different pressures, generally a time between 1.5 and 2 is sufficient to obtain reliable results with errors less than 5%. We have also found that this equation describes the desorption process as well.