The contents of two recent papers concerned with a thermodynamic analysis of interference effects in ion selective membrane electrodes are summarised and combined using a slightly different but equivalent formalism. The discussion focuses on the behaviour of the operational interference potential, E+, and the variables on which it depends. A general limiting expression for the dependence of E+ on solution composition when E+ is small is highlighted and a calibration procedure enabling interference effects to be allowed for in a way that is model independent is described. This procedure is not confined to situations where E+ is small. In principle it enables reliable thermodynamic data to be obtained using electrodes that suffer from interference and by so doing extends the scope of EMF measurements as a tool for obtaining such data. A summary of some experimental results on Ca++ selective membranes that exhibit interference from Na+ and dodecylsulphate (DS-) is presented. The observed interference effects are in good accord with the predictions of the new theory.