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This article introduces a method to validate heat of sorption measurements from isotherms by comparing them to direct measurements using reaction calorimetry. We have evaluated some frequently used single-temperature isotherm models (BET, GAB) as well as two multi-temperature isotherm equations (Anderson, Heikkilä). It turned out that the isotherm models with better isotherm curve fitting characteristics did not deliver the best results for the heat of sorption. Multi-temperature isotherm models such as the Heikkilä equation are able to deliver good results in terms of modelling the isotherm data and determining the heat of sorption. Single-temperature models fit the isotherm data better than multi-temperature models, but failed to deliver correct values for the heat of sorption. We specifically investigated the heat of sorption at very low moisture content, which is most relevant for paper drying. The resulting heat of sorption curves were analyzed with respect to their saturation behavior, which is expected to occur below BET monolayer moisture. The heat of sorption from the Heikkilä’s model was the only one to show the expected saturation at very low moisture content. [ABSTRACT FROM PUBLISHER]