Solubilities of the molecular compound of sulfanilamide and sulfathiazole in aqueous solutions containing varying amounts of sulfanilamide were determined. It was found from the results that the solubility product principle can be applied to such mixtures. Based on the principle, a method for calculating the value of the saturated concentration of the compound was proposed. Besides, the influence of each one of the sulfonamides to solubility of the other was investigated. Using all these data, the phase diagram of the system of sulfanilamide, sulfathiazole and water was constructed which indicates that the system belongs to type where a one-to-one molecular compound is formed but is decomposed by water. It was also confirmed that the metastable solubilities of the compound observed in water and in the sulfanilamide solution can be represented by the points on the extention of the saturation curve of the molecular compound in the diagram.