A new miniaturised reference element was realised by combining a solid ion reservoir with a polymer membrane for diffusion restriction. Different polymer membranes were investigated by measurements of impedance, permeation and sensitivity to select an appropriate membrane material that meets the necessary requirements. These include water uptake, effective diffusion restriction, low electrical time constant and no pH sensitivity. Membranes of cellulose-2,4-acetate (CDA) were found to combine these properties. The formation of diffusion potentials and their time dependence were examined for their dependence on the electrolyte and concentration. A semi-quantitative theoretical description is proposed. Because of the great water uptake of CDA membranes the diffusion potential does not disturb the reference function of the sensor in permeable electrolytes.