Addition of ethanol to propellant based formulations of pMDIs results in saturated vapour pressure alteration which is understood to have an impact on the droplet size of the resulting aerosol cloud 1, 2, 3, 4. Accurate and simple methods of estimating the saturated vapour pressure are important in developing the next generation of pMDI formulations. Here we report the results of a series of vapour pressure measurements of ethanol/HFA134 and ethanol/HFA227 binary mixtures over a wide range of ethanol concentration and temperature. The outcome shows strong departure from Raoult’s law. An empirical expression is developed to capture the nonlinear behaviour of vapour pressure in a compact set of equations…