Application of silicon based microsensors in electrolyte solutions is hampered by insufficient barrier properties and poor corrosion resistance of common passivation layers used to protect the underlying conducting tracks and microelectronic structures. Therefore, the protectivity of various types of compatible passivation layers (organic polyimide and photoresist films, inorganic mono, duplex and tripler layers based on PECVD silicon oxide and silicon nitride) was investigated and improved on microelectrode arrays exposed to 1 M NaCl (pH 2 to 10) at 25 degrees C. Duplex SiO2/Si3N4 and oxide/nitride/oxide (ONO) triplex layers with optimised nitride PECVD process yielded the best barrier properties. Burying the conducting tracks in the thermal silicon oxide layer improves the performance significantly Failures of the passivation layers, detected by leak current and layer resistance measurements with subsequent SEM investigation, result from cracking due to intrinsic and extrinsic (less important) mechanical stress, film defects (pinholes, particle inclusions), from chemical, physicochemical and electrochemical reactions (external, internal, sublayer corrosion) and from the combined action of mechanical stress and chemical interaction (stress corrosion cracking).