In electrochemical sensors like pH-, reference- or ion-selective electrodes a porous ceramic plug (or diaphragm) maintains the conducting junction with the test solution. These liquid junctions should have a resistance as low as possible meanwhile avoiding leakage through the junction. Five porous magnesium stabilised zirconium oxide plugs with different porosity's and pore size (distribution) were investigated as liquid junctions. The physical properties of these porous plugs were investigated with SEM and Mercury Intrusion Porosimetry. Important working conditions of these porous plugs are the resistance of the porous plug filled with an electrolyte and the contamination speed through these porous plugs, both for the test solution as the reference solution. The first property was measured by a 4-wire resistance measurement. The second property was measured by measuring the flow through rate of the reference electrolyte through the plug. It was shown that an optimal plug i.e., low leakage and high conductivity through the membrane, had a high porosity and relative small pores (0.25 mum). A simple mathematical model based on the Hagen-Poiseuille equation was developed to describe the porous plug characteristics. It was shown that mathematical calculation of the porous plug resistance was in good agreement with experimental results.