Moisture sorption isotherms of pistachio nut paste were determined using the gravimetric static method of saturated salt solutions at 10, 20 and 30 degrees C. Isotherms were found to be of type III. The effect of temperature on equilibrium moisture content values was not significant (P > 0.05). The sorption isotherms did not exhibit hysteresis over the whole a(w) range. At higher water activities the moisture content increased sharply as the temperature was increased, resulting in a crossing of the isotherm curves. Five models available in the literature, namely the BET, GAB, Halsey, Henderson, and the Iglesias and Chirife, were evaluated to determine the best fit for the experimental data. The BET model used for data in the lower a(w) range (less than or equal to 0.43) was found to be adequate and the Halsey model superior for characterising the sorption behaviour of pistachio nut paste in the temperature and water activity range investigated. The usual procedures for calculating isosteric heats of sorption by the application of the Clausius-Clapeyron equation was examined. The isosteric heats of sorption increased with decreasing moisture content.