Vapour pressures and equilibrium liquid–vapour mole fractions have been determined for the solvent system ethanol–acetone at 32°, 40°, and 48 °C. in a modified form of the apparatus developed by Ferguson and Funnell. It is shown that the data are consistent with the Gibbs Duhem equation if it be assumed that deviations from ideality in the vapours may be computed from the principle of Corresponding States. It is also shown that the variation of the activities of the components in the liquid with temperature is in moderate agreement with the heat of mixing data for the system.