The desorption isotherms of apples of the Granny Smith variety, treated with 0.5% citric acid aqueous solution, were determined using the gravimetric static method. Experiments at three temperature levels: 20°C, 40°C and 60°C and ten water activity levels from 0.112 to 0.856 were carried out. Along with five well known models - modified Chung-Pfost, modified Halsey, modified Oswin, modified Henderson and GAB (Guggenheim-Anderson-de Boer), a new fraction-linear model has been applied to fit the data and to calculate the isosteric heat of desorption determination.