Moisture sorption isotherms of cowpea (Vigna unguiculata L. Walp), powdered cowpea and protein isolate of cowpea at 10, 20 and 30 degrees C were derived. The sorption data were treated according to many well-known sorption isotherm equations. The goodness of fit was evaluated on the basis of criteria such as the mean relative deviation modulus, coefficient of determination and standard errors of models. It was found that the GAB model was the most satisfactory model for representation of the sorption data. Monolayer moisture contents (MO) were determined from the GAB and the BET equations. The temperature dependence of Mo was examined. Heats of sorption data were evaluated using the Clausius-Clapeyron equation. An attempt was made to determine the moisture contribution of the protein component of cowpea to the total moisture content. (c) 2004 Elsevier Ltd. All rights reserved.