The decomposition of hydroquinone and of 2,5-di-tert-butylhydroquinone in dichloromethane solutions has been studied by in situ UV absorption and by in situ infrared reflection-absorption spectroelectrochemical methods. The UV absorption spectra showed that benzoquinone and 2,5-di-tert-butylbenzoquinone are respective products of these decompositions. The infrared spectra confirmed that the quinones were the principal reaction products but also indicated that some water was formed during the decomposition processes. The decomposition rates were found to be dependent on electrode potential with a minimum rate corresponding to a maximum coverage for adsorbed hydrogen, a product of the decomposition. A new infrared absorption at 1713 cm(-1) has been detected at positive potentials in the hydoquinone system and has been ascribed to endwise adsorbed benzoquinone.