The response of miniaturized amperometric sensors depends heavily on the thickness of the diffusion-limiting membrane. Therefore, on-wafer processing of complete devices is an attractive technique to produce amperometric sensors with a reproducible and well-controlled sensitivity. In this paper we present a method for the deposition and photolithographic patterning of polyHEMA hydrogel membranes in the thickness range 10-100 μm. The attachment of these membranes to the substrate is ensured by a chemical pretreatment of the substrate surface. The sensor behaviour is evaluated in solutions containing Fe(CN)64- and H2O2, and compared to that of the bare electrodes. The membrane appears to be impermeable for Fe(CN)64-, whereas the complete sensor gives a linear response to H2O2. Furthermore, the possibility of using the structure as an oxygen sensor is evaluated.