Although various methods have from time to time been described for determining the molecular weights of dissolved substances by measurement, not of boiling-point elevation, but of the reduction of vapour pressure of a solvent due to the presence of a dissolved substance, none of these methods has come into general use. The method here proposed, operating on this principle, offers the possibility of simply determining the molecular weights of non-volatile solutes in any of the ordinary solvents with an accuracy at least equal to that of the ebullioscopic methods, and in an apparatus at least as easily manageable.