The aim of the present work was to study the solubility and phase behaviour of the β-sitosterol–cholesterol mixed crystals in the presence and absence of water. Cholesterol, β-sitosterol and 3:1, 1:1 and 1:3 mixtures of these were co-precipitated from acetone and acetone–water solutions. Precipitated crystals were analysed using powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), optical microscopy and Karl–Fischer titrimetry. The quantification of the sterols in solutions was preformed using GC–MS. The solubility of the sterols was mutually limiting. In the aqueous system, the solubility of both the sterols were significantly lower than in the absence of water, but the decrease in the solubility was considerably greater with the more hydrophobic β-sitosterol. In the aqueous system, the total sterol solubility decreased with the increasing proportion of β-sitosterol. The formation of new crystal structures, solid solutions of cholesterol and β-sitosterol, was observed in non-aqueous as well as in aqueous environments except with the lowest cholesterol proportion in the system, in which case mixed crystals with eutectic behaviour were formed.