The phase behavior of ternary blends of bisphenol-A polycarbonate (PC), tetramethyl bisphenol-A polycarbonate (TMPC), and poly (epsilon-caprolactone) (PCL), where each binary pair forms completely miscible blends, was investigated. The ternary blends were found to be miscible for all compositions and do not phase separate prior to thermal decomposition. The melting point depression method based on both the Flory-Huggins theory and the equation of state theory of Sanchez-Lacombe was used to evaluate interaction parameters for each pair.