We present direct measurements of the partial molar enthalpy of carbon dioxide sorption in a family of structurally related bisphenol-A based polymers. Polymers included in this study are : bisphenol-A polycarbonate, tetramethyl polycarbonate, tetrachloro polycarbonate and tetrabromo polycarbonate. All samples were conditioned at 300 psig prior to measurement. Enthalpy changes were determined using a Setaram C80D microcalorimeter equipped with high pressure cells. The magnitude of the heat of sorption does not correlate well with polymer free volume, gas solubility or gas partial molar volume. However, the values are consistent with heats of sorption calculated from the temperature dependence of solubility. The results suggest that free volume regions are not uniformly accessible; occupation of some free volume regions requires unfavourable deformation of the glassy polymer matrix. The measured heat of sorption represents an energetically weighted average of those free volume regions which are occupied.