The applicability of ion selective membrane electrodes is often limited by interference effects from species other than that one wishes to monitor. Such effects can be described in terms of an interference potential, E+, which is the difference in the measured electrode response in the solution of interest minus that which would be observed in a solution where the activity of the species to be studied is the same but which contains no interfering Species. Previous work, based on thermodynamic arguments, that identifies the variables that determine E+ is amplified. This leads to calibration procedures that enable E+ in an unknown solution to be determined from appropriate measurements using known solutions and also to criteria that must be met for these procedures to apply. In contrast with previous work, no models for equilibria and transport processes in the membrane interior are required. The method is extended to species involved in multiple dissociation equilibria such as polyacids and polyamines.