The compatibility of the poly(ethylene oxide)-atactic poly(methyl methacrylate) (PEO-aPMMA) system has been studied using Patterson’s theory of polymer-polymer miscibility. Specific volumes and thermal pressure coefficients for both polymers have been accurately determined; the characteristic parameters V*, T* and P* are obtained from the experimental results. Experimental heat-of-mixing data were used to calculate the interaction energy between the two components. The shape of the curve Of chi12/V1* versus temperature indicates that PEO-aPMMA mixtures are miscible in the liquid state over all the composition and temperature range explored.