Properties of compatible multicomponent polymer systems can be studied quantitatively only if their fundamental thermodynamic parameters can be established. Thus measurements of mixing enthalpies in such polymer-polymer pairs is of interest 

in arriving at criteria for compatibility. Direct determinations are awkward, but the required parameters can be obtained from differential heat of solution measure­

ments. The metastability of polyrrer blends in phase separated regions below their glass transition temperatures provides additional insight when calorimetric measures 

are performed below Tg. These factors are illustrated with examples of compatible, near-compatible and incompatible binary mixtures containing poly(2,6-dimeth phenylene oxide) and polystyrene or halogenated derivatives of polystyrene.