A simple equation for calculating the solubilities of solid substances in gaseous mixtures of a supercritical fluid with another supercritical one or an inert gas was derived. This equation involves only the solubilities of the solid in the individual constituents of the gaseous mixture and the molar volumes of the latter. The equation was tested for the solubilities of naphthalene, benzoic acid, caffeine, cholesterol, and soybean oil in gaseous mixtures containing at least one supercritical fluid. In all cases good agreement with experimental data was found.