This research studied the moisture sorption behaviour of jatropha seed allowing the development of a model correlating the equilibrium moisture content (EMC) and the free fatty acids (FFA) content as a function of water activity and EMC, respectively. Two sets of sorption-isotherm experiment were performed to describe the relationship between EMC and FFA content with water activity, for both fresh and dried seeds. The seeds were conditioned in series of saturated salts having certain water activity and stored at different temperatures (20, 30, 40 C). The nested experimental design and ANOVA (F-test at p = 0.05) were applied to study the effects of temperature and water activity on the EMC of jatropha seed. EMC increased generally with the increase in water activity and with the decrease in temperature at either desorption or adsorption. The EMC was significantly affected by water activity, but the temperature did not significantly affect the EMC. The hysteresis effect was more pronounced at lower temperatures. The FFA content of seed was relatively constant at low water activity (a(w) < 0.8), but it was high at higher water activity. The moisture sorption behaviour of jatropha seed revealed that like most product, it exhibited the sigmoid pattern. The BET, GAB, Harkins-Jura, Halsey and Henderson models were recommended as adequate models in predicting the amount of moisture adsorbed or desorbed at known humidity. The relationship between EMC and FFA content of jatropha seed showed that the polynomial equation was the best fitting for either fresh or dried seed. (C) 2011 Elsevier Ltd. All rights reserved.