“PEQ” is a QuickBASIC program package for the construction of mineral phase equilibrium diagrams such as P-T, T-XCO2, P-XCO2, lnfO2-T and μx-μy diagrams using a personal computer. The thermodynamic datasets for minerals of Helgeson et al. (1978) and Holland and Powell (1990) are incorporated into the program. The non ideal mixing of H2O and CO2 can also be taken into account. Some examples of the construction of phase diagrams using “PEQ” are given.