A thermodynamic relationship is used to describe how the presence of an impurity affects measured vapor pressures by relating the effect to the distribution coefficient, K, of the impurity. In practical situations K is estimated simply by analysis with a gas chromatograph. A second relationship is used to describe how K, and thus the effect, varies with temperature. The effect of azeotropic behavior on the temperature variation is also considered. Several examples are given, including the systems CH2F2 + CF3CH2F (HFC32 + HFC134a), CF3CF2Cl + CF3CHF2 (HCFC115 + CFC125), CHF2CF2CH2F + CF3CF2CF2CH2F (HFC245ca + HFC338mccq), and samples of CF3CH2CF3 (HFC236fa) and n-heptane with impurities. (C) 1998 Elsevier Science B.V. All rights reserved.