Moisture sorption dynamics and isotherms of fresh, osmotically-pretreated and dried papayas at temperatures of 30, 50 and 70 degrees C and water activity in the range of 0.113-0.907 were investigated. Chemical composition as well as volume, density, porosity and microstructure of the fruits were analysed. Results showed that the time required to reach equilibrium moisture content was mainly dependent on temperature, water activity level and processing method. The difference in moisture sorption characteristics between fresh, pretreated and dried papayas was attributed to (i) changes in the contents of sugars after osmotic dehydration and (ii) structural modifications caused by drying, which were corroborated by examination of micrographs. The differences in sorption were also proven by the selected isotherm models. The modified Halsey equation described the sorption behaviour of the pretreated and dried papayas, whereas the untreated, fresh samples were better predicted by the modified Oswin equation. Further indication that osmotic pretreatment and drying influenced the interaction between the sorption sites and water molecules was imparted by the values of isosteric heat of sorption (Q(st)) for fresh and dried papayas at different temperatures. (C) 2015 Elsevier Ltd. All rights reserved.