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ABSTRACT*The thermoelectric vapor pressure osmometer is established as a satisfactory instrument for the determination of the vapor pressure lowering of solutes in solutions. The calibration constant, a 1 , of the instrument is demonstrated to be independent of the nature of the solute and can be calculated from a knowledge of the system geometry and measurable thermodynamic and transport parameters.Experimental results are evaluated in terms of the change in resistance of the thermistor and the concentration of solution. It is necessary to use a seconddegree equation in concentration plot to fit the data. Evaluation of the coefficient of the second-order term requires the incorporation of a heat-of-mixing term in the form of the Van Laar equation. The concentrationindependent term 8 for the heat of mixing is obtained for fourteen solute-solvent pairs.