Vapour pressures of normal water, water enriched by the heavy isotope of oxygen (H2O)-O-18, and saturated aqueous solutions of KCl were determined in the temperature range (298 to 318) K by the isoteniscopic method and compared with the literature data. Water activities, osmotic coefficients, vapour pressure lowerings, boiling point elevations, and molar enthalpies of vaporization at the saturation point of potassium chloride were evaluated from the measured vapour pressures.