With a view to the miniaturization of ion-selective electrodes (ISEs), thin (10−20 μm) polymer membranes are directly contacted to Au covered with a redox-active, lipophilic self-assembled monolayer (SAM). Several homogeneous and mixed monolayers are characterized by reflection−absorption infrared spectroscopy, ellipsometry, scanning tunneling microscopy, cyclic voltammetry, and contact angle measurements. These Au/thiol surfaces are combined with different K+-selective sensing membranes based on poly(vinyl chloride) (PVC), polyurethane (PUR), or PVC/PUR blends as a matrix and valinomycin as an ionophore. The sensors are investigated with regard to their potential stability in the presence of O2 and redox-active species. The occurrence of potential drifts upon changing the conditioning KCl solution to a NaCl solution is used as an indicator for the formation of an aqueous film between the membrane and Au/SAM. Stable systems are obtained with mixed monolayers (advancing contact angle θa ≈ 83°) and PVC memb...