A modification of the GAB isotherm (Guggenheim-Anderson-De Beer) is proposed in order to correlate the sorption data for water activities higher than 0.9. The proposed isotherm retains the desirable properties of the GAB isotherm, i.e. good fitting in the range of a, between 0.05 and 0.80, and also provides a noticeable improvement in the fitting quality for high values of a(w) by introducing a new term with an additional constant. Sorption data for native potato starch and gluten at different temperatures (2 degrees C, 20 degrees C, 40 degrees C and 67 degrees C) were well correlated far the whole a(w) range. For starch at 67 degrees C, the values of the constants of the proposed equation do not follow the same tendency obtained for lower temperatures, suggesting that the structure of the material could be changed due to the high temperature. The proposed isotherm can be of interest in the area of drying given that there are few isotherms that accurately represent sorption data at different temperatures in the zone of high a(w). Also, it can be useful to predict other thermodynamic functions. In addition, a new procedure is proposed to determine the isosteric heat by using a second order polynomial for representing the variation of moisture as a function of temperature at a fixed water activity.