The effect of the fractal surface on physical adsorption has been analysed and a new three-parametric isotherm equation was derived. The model describes the multilayer adsorption on fractal surfaces within the framework of BET theory. It predicts lower buildup of the multilayer coverage on a fractal surface than on a flat surface The results of the simulation show that increasing the fractal dimension of the surface affects the adsorption isotherms; namely, the upward curvature of the isotherms at high relative pressures is decreased as D increases. For the submonolayer region, the curves become independent of the fractal nature of the absorbent. The isothermal data for water vapor sorption on grains and other starchy materials were used to evaluate the new isotherm equation. The parameters of the isotherms were calculated using computational regression techniques. Fractal dimension ranged from 2.6 to 2.9, approximately, indicating the contribution of the pore structure to the sorption capacity of the products investigated. Results show that the model allows an accurate and simple description of the sorption data for the whole range of water activity.