SUMMARY— Several existing mathematical models are investigated with respect to their range of applicability to biological materials. It is found that a logarithmic plot of vapor pressure of water in the material vs. vapor pressure of pure water can best describe the sorption phenomenon. The general form of the equation is: In P = f1 (W) In Ps+ f2 (W). The functions f1 (W) and f2 (W) may differ significantly from material to material. The method of determining these functional relationships is described in detail for rough rice and peanuts. The comparison of experimental and computed values for peanuts shows that this method can predict the required sorption isotherm data within 2% deviation from the experimental values.