Flow microcalorimetry allows measurement of thermodynamic quantities that are associated with interactions between solvent molecules and the structural features of polymeric materials. The type of information that can be obtained when a hydrocarbon vapor (n-butanol) interacts with the perfluorosulfonate ionomer, Nafion(TM), is described. The molar enthalpy of sorption was determined for the untreated polymer and the residual polymer after extraction by aqueous propanol. The results for the untreated polymer are compared with the interaction between water vapor and Nafion(TM) over a wide range of relative pressure.