Transition metal oxides with strong electron correlation show a variety of unusual properties. Establishing the electronic and magnetic phase diagrams as a function of composition and temperature is a first step in understanding their behavior, and sometimes a crucial step in discovering novel properties. This article reviews phase-diagram studies on RCoO₃, RMnO₃, RMn₂O₅, and Y_{2 －x} Bi_xRuO₇ (R ＝ rare earth), where heat capacity measurements were instrumental in identifying the phase transitions and understanding the complex properties. High-pressure synthesis and synchrotron powder x-ray structural analysis also played important roles in constructing and understanding the phase diagrams.