Thermodynamics and molecular mechanisms of sorption of gases and liquids by glassy polymers and are still not fully understood. In particular, the enthalpy of sorption (or mixing) in the glassy state - a parameter crucial for understanding the thermodynamics of sorption is not properly described by existing approaches. In this work, we propose a thermodynamic theory that describes the effect of the glass transition on the enthalpy of sorption. Firstly, a rigorous thermodynamic expression for the sorption enthalpy is presented, and then equations applicable for practical calculations are derived using certain approximations. The theory presented here is tested on the experimental water sorption data on starch and cellulose. The equations describing the hydration enthalpy are in excellent agreement with experimental data. Remarkably, the glass transition-induced apparent heat capacity change for water in carbohydrate polymers turned out to be negative. Being counterintuitive, this result can however be supported by re-evaluation of the literature data on heat capacities of the starch-water system.