Electrocatalytic films of mixed (substitutional) nickel-iron hexacyanoferrate with the Ni/Fe molar ratio up to 0.13 were prepared by incorporation of Ni2+ ions into the structure of electrochemically deposited thin Prussian blue films. The obtained films retained the high electrocatalytic activity inherent to Prussian blue and were stable towards degradation in alkaline media up to pH 9.3, enabling the sensing of 5 x 10(-8) to 2 x 10(-4) mol/dm(3) of H2O2 at pH 9.0, with the sensitivity of 1.14 AM(-1) cm(-2) and the detection limit of 2 x 10(-8) mol/dm(3). Based on the results of electrochemical. XPS, and SEM investigations, the stability of mixed nickel-iron hexacyanoferrate films at pH 9.0 was attributed to the hydrolysis-induced lattice transformation occurring at the film/solution interface and resulting in the formation of a thin and compact layer of an alkaline-stable nickel hexacyanoferrate, which covers the remaining nickel-iron hexacyanoferrate film and protects it from further degradation. (C) 2012 Elsevier Ltd. All rights reserved.