This paper describes the successful use of thin layers of birnessite to degrade methylene blue (MB) contained in aqueous solutions. This dye is a representative of a class of dyestuffs resistant to biodegradation. The thin layers of birnessite, electrodeposited onto a cheap transparent semiconductor substrate (SnO2), in neutral aerated sulphate solutions at room temperature, are very homogeneous, adherent and well crystallised. We showed that these thin layers can discolor the MB solutions totally at room temperature after only 2 h. Total organic carbon (TOC) analyses report the absence of organic compounds in solutions after treatment, in the best conditions. Mass spectrometry reveals the presence of intermediate reaction products (azure A. azure B, azure C, and thionin) during interaction confirming the real degradation of dye. Nitrate and ammonium ions were quantified showing that thin layer of birnessite can mineralise MB. To understand mechanism, interaction experiments were made with each intermediary compound. The electron transfers lead to a rapid N-demethylation oxidative reaction to form colored intermediary compounds. In all cases, thionin was identified as the last adsorbed organic compound onto the birnessite external surface, which is then mineralised slowly into nitrate and ammonium ions. These results suggest that birnessite thin layers may be envisaged as a new non-toxic material for treatment of diluted colored wastewater. (C) 2010 Elsevier B.V. All rights reserved.