The fabrication of chemical transducers using standard Si/SiO2/metal microelectronic technology has given rise to a widespread development of ultramicroelectrode arrays (UMEAs) as analytical tools. The electrochemical behaviour and performance of UMEAs depend on their geometry, radius and inter-electrode distance. Therefore, a suitable design and fabrication process is required. Provided that UMEA fabrication processes are not totally efficient so far, a small fraction of the electrodes on-chip always remain passivated. In this work, Pt and Au UMEAs with three different geometries were designed and fabricated. A complete electrode characterization in terms of fabrication efficiency and analytical response is carried out by combining electrochemical techniques, transmission light and fluorescence optical imaging. Voltammetric measurements performed in ferrocyarride solutions used as a model electroactive molecule, always results in a reproducible sigmoidal response for different potential scan rate intervals depending on the electrode geometry. Optical visualization of the electrodes using transmission light microscopy relies on either electrochemical oxidation of the electrode surface or electrodeposition of gold nanoparticles. Alternatively, the anchorage of fluorescent oligonucleotide conjugates on the gold nanoparticle surface enables electrode imaging by fluorescence microscopy. Results obtained with the different techniques proposed show an excellent correlation among them. Differences in data turned out to be non-statistically relevant. It is demonstrated that less than 20% of the individual electrodes within one chip remained passivated. (c) 2007 Elsevier Ltd. All rights reserved.