A numerical method for determining solute diffusivity in polymers over wide temperature ranges was developed based on non-isothermal thermogravimetry (TG). The method combines the Fickian equations with a heat transfer equation in order to take into account temperature gradients within the polymer sample during a heating scan. The requirement of thermal equilibrium in the specimen during the heating scan need not be fulfilled, and this permits the study of desorption of solute from thick specimens at high heating rates. The numerical procedure is capable of considering concentration-dependent solute diffusivity and temperature-dependent thermal diffusivity. The method was tested on the desorption of n-hexane in polyethylene between 25 and 120 degrees C. The results obtained agreed with experimental data obtained at heating rates from 10 to 40 degrees C/min up to 90 degrees C and yielded an activation energy of 46 kJ/mol, which is in agreement with earlier reported experimental data.