ABSTRACT The thermodynamic fundamentals of fluids and solids are shortly reviewed, and the criteria of phase equilibria discussed. The structure of the versatile thermoC program package is outlined, and its working demonstrated for several examples. It is shown that complete phase diagrams involving solid-fluid and fluid-fluid phase equilibria can be predicted with a single parameter set, and that it is even possible to estimate pure-compound sublimation data from binary solid-fluid high-pressure phase equilibria. Several mixing theories are applied to the correlation of the gas-gas equilibria of the (neon + krypton) system. It is shown that only mixing rules with density-dependent fractal exponents can achieve a quantitative agreement.