Ternary blends comprising bisphenol-A polycarbonate (PC), the polyhydroxyether of bisphenol-A (Phenoxy), and p l y ( c-caprolactone) (PCL) were found to be generally miscible at PCL levels greater than 60% by weight and to show multiple amorphous phases at lower PCL levels. The melting point depression of PCL in the miscible region of the ternary and in the miscible binary blends with PC and Phenoxy was examined to obtain the enthalpic interaction parameters, Bi,, for each of the three binary interactions. The parameters associated with the miscible binary blends were negative, as expected, and indicated that PCL interacts more exothermically with Phenoxy than with PC. The parameter associated with the Phenoxy/PC interaction was strongly positive as expected from the complete immiscibility shown by these materials. The interaction parameters were used to calculate the locus of compositions for which the heat of mixing is zero. The locus was found to agree well with the observed boundary between miscible and multiphase behavior in the ternary. This suggests that the phase behavior of ternary blends is largely determined by the same enthalpic considerations known to govern the phase behavior of binary blends.