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Enthalpy changes (Δ
H
tot) on the immersion of dehydrated human serum albumin (HSA) into water–dioxane mixtures have been measured using a Setaram BT-2.15 calorimeter at 298
K. Thermodynamic activity of water was varied from 0 to 1. Calorimetric results are discussed together with the FTIR-spectroscopic data on water and organic solvent vapor adsorption/desorption isotherms on solid HSA. Dioxane sorption exhibits a pronounced hysteresis. Calorimetric and dioxane desorption dependencies consist of two parts. No dioxane sorption was observed in low water activity region (
a
w<0.5). At low water activities, the Δ
H
tot values are close to zero. At water activity about 0.5 the sharp exothermic drop of the interaction enthalpy values was observed. This exothermic drop is accompanied by the sharp increase in the amount of sorbed dioxane and additional water sorption (compared with that for pure water). Dioxane adsorption branch resembles a smooth curve. In this case, solid HSA binds more than 300
mol dioxane/mol HSA at low water activities. By using a water activity-based comparison we distinguished between dioxane-assisted and dioxane-competitive effect on water sorption. The obtained results demonstrate that the hydration “history” of solid protein is an important factor that controls as the state of protein macromolecule as well as the sorption of low-molecular organic molecules.