Carbon dioxide sorption and dilation data were fitted with the glassy polymer lattice sorption model (GPLSM) to obtain segment-segment interaction parameters for poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), polystyrene (PS), and two PPO/PS miscible blends. These interaction parameters were then used to calculate the enthalpy of mixing for the two polymers. The resulting enthalpies were -0.2 and -1.1 cal/cm(3) for PPO/PS blends of 50/50 and 75/25 by weight, respectively, which compare favorably to those determined from measured heats of solution and from calorimetry with low molecular weight analogs.