Electrochemical studies of both the a-Si:H based metal/oxide/semiconductor structures and a-Si:H modified Pt electrode as a part of two-electrode cell compartment with electrolyte were performed to obtain information on the nature of mobile hydrogen in a-Si:H. We assume, that the complex formed by a hydrogen molecule and a silicon dangling bond (the origin of D-h states) provides for mobile hydrogen. Information on the process of electron selfexchange (hopping) in a-Si:H driven by diffusion in the presence of a concentration gradient of redox centers is obtained. The capability of the electrochemical methods for studying the role of hydrogen in degradation of a-Si:H caused by bias annealing and/or illumination is demonstrated. Also, it was shown that because of the unique electrocatalytic features of the a-Si:H modified Pt electrode, the latter can act as a "programmable" and selective sensor of heavy metal ions and/or metal complexes. (c) 2006 Elsevier Ltd. All rights reserved.