The formation energetics and heats of mixing of lead zirconate titanate (PZT) PbZr x Ti 1ุx O 3 solid solutions have been investigated using high-temperature oxide melt solution calorimetry. While the heat content (H 973 ุH 298 ) is almost constant (80+82 kJ/mol) for x โซุโฌ 0.15+1.0, its value for PbTiO 3 (x โซุโฌ 0.0) is larger (86.15$0.55 kJ/mol). Linear variation in the enthalpy of drop solution, H ds , and the enthalpy of formation from the oxides, H f,ox (298 K), is observed in the tetragonal region (0< x< 0.5) and continues smoothly into part of the rhombohedral region (0.5< x< 0.65). Fitting the data in the tetragonal phase region (0< x< 0.5) to a straight line enables an extrapolation to give the heat of the virtual transformation PbTiO 3 (tetrag-onalP Porthorhombic) as 24.0 ุ 3.9 kJ/mol. An extrapolation from the rhombohedral region gives 33.3 ุ 3.6 kJ/mol for H of the virtual transition PbZrO 3 (orthorhombic or rhombohedral P Ptetragonal). The transition enthalpy between orthorhombic and rhombohedral PbZrO 3 is zero within experimental error. The enthalpy of formation from the oxides, H f,ox (298 K), varies from ุ36.7 ุ 2.7 kJ/mol for PbTiO 3 to 0.7 ุ 4.3 kJ/mol for PbZrO 3 . The H f,ox (973 K) values agree with previously reported H f,ox values measured in alkali borate solvent at 1073 K. Fitting H mix (973 K) to a regular solution model yields an interaction parameter, W, of 39.8 ุ 4.6 kJ/mol. However, structural complexity makes the regular solution model only a crude physical description.