A measurement of equilibrium moisture of the agricultural products by gravimetric methods can be accomplished by well-known procedures usually consuming a lot of time essential for reaching equilibrium state between the tested object and conditioned environment. In this article we describe the measurement results of equilibrium moisture of wheat, barley and maize grains obtained during desorption process in conditioning chamber providing a stabilised climate of drying space. Regression equations for drying curves w = f(t) and also from this derived isothermal lines wr = f(f) valid for equilibrium moisture obtained with use of usual gravimetric method and the proposed reduced gravimetric method are presented here. The applicability of conditioning chamber Feutron - 3001 used for measurement of grain equilibrium moisture was proved with sufficient probability by mathematical statistical analysis of the measured results. The verified reduced method offers results differing in compare to the control usual gravimetric method in moisture less than 1 % and consuming only 14.84 % - 10.32 % of testing time according to the permissible measurement error 0.25 % - 0.5 %.