Electrodeposited layers of Prussian blue (PB) and electrocatalytic reduction of hydrogen peroxide at electrodes, modified with these layers have been studied with in situ Raman spectroelectrochemical technique. Characteristic Raman features have been observed and attributed to reversible electrochemical transition between the oxidized (PB) and reduced (Prussian white, PW) forms of the modifier. It has been shown that, during the cathodic reduction of hydrogen peroxide, PW layer turns partially into its oxidized form PB even at electrode potentials corresponding to the reduced form of a modifier. The ratio of PB/PW within the modifier layer has been shown to depend on peroxide concentration, indicating that electrocatalysis proceeds within the modifier layer rather than at an outer modifier/electrolyte interface. As opposite, electrooxidation of ascorbate does not affect in situ Raman spectra and thus proceeds most probably at an outer interface. The kinetics of PB decomposition during electrocatalytic reduction of hydrogen peroxide has been studied by in situ Raman spectroscopy, and the corresponding first-order reaction rate constant has been obtained. (C) 2011 Elsevier B.V. All rights reserved.