In a binary mixture, partial vapor pressure may show either a positive or negative deviation from the predicted value of an ideal solution. In this report we derive the deviation from Raoult's law from the heat of mixing, ΔHmix, and the molal volume, V, of each of the components of a binary solution. This derivation is then tested for seven sets of combinations of two different solvents, taken at random from the literature. Each set consists of several different ratios of solute‐solvent. The correlation between the reported experimental values of the partial vapor pressure of a given component, P1, and the theoretically derived values is excellent. The same derivation is further applied to calculate the solute‐solvent interaction parameter, β12, independently from the geometric mean assumption of regular solution theory. In a number of cases, especially in hydrocarbon‐alcohol mixtures, β12 proves to be significantly different from the calculated geometric mean or from the Walker interaction parameter term, K.