A thin aqueous layer is formed between the polymeric ion-selective membrane and an inner gold electrode. Its composition can be altered upon sample changes, which leads to drifting potentials. It is shown by theory and experiment that interfering ions enter the inner layer much faster than primary ones. Measurement protocols with equal contact times for the sample and the reconditioning solution can therefore not eliminate the effect of changes in the composition of the inner solution. The formation of an inner aqueous film and corresponding drifts for solid-contacted membrane electrodes can be avoided by creating a lipophilic self-assembled monolayer on the surface of the inner electrode.