Direct measurements of the partial molar enthalpy of carbon dioxide sorption in polycarbonate and poly(dimethylsiloxane) are reported. Sorption is a highly exothermic process. The heat of sorption in poly(dimethylsiloxane) is constant, similar to-1.5 kcal/mol, up to 300 psig, and the value does not depend on whether the measurements are made during pressurization or depressurization. For polycarbonate, however, the magnitude of the enthalpy change is much greater, varying from -3.5 to -5 kcal/mol. This increase is attributed to the presence of excess free volume in the glassy polycarbonate, which makes sorption energetically comparable to vapor condensation. If measurements are made during depressurization immediately after pressurization, the partial molar enthalpy of sorption increases dramatically. We attribute this behavior to gas plasticization effects during pressurization since preconditioning the sample at the maximum exposure pressure eliminates the difference. The magnitude of the partial molar enthalpy of sorption increases if the sample is preconditioned at increasing carbon dioxide pressures. Furthermore, the value is constant over a broad pressure range, from 0 to similar to 300 psig. A constant value is not consistent with a multiple-site sorption mechanism unless the heat of sorption is the same for all sites or the relative occupancy of each site remains constant.