A series of transition metal substituted polyoxometalates (POMs) have been attached to carbon electrodes in multilayer films of poly(allylamine hydrochloride) (PAH). These include Wells-Dawson-derived sandwich-type Fe(4)(III)(H(2)O)(2)(As(2)W(15)O(56))(2)(12-)center dot CO(4)(II)(H(2)O)(2)(As(2)W(15)O(56))(2)(16-) and Mn(4)(II)(H(2)O)(2)(As(2)W(15)O(56))(2)(16-) and Wells-Dawson mono-substituted-type alpha(1)-[P(2)W(17)V(IV)O(62)](8-) and alpha(2)-[P(2)W(17)O(61)(OH)(2)Fe(III)](7-). The modified electrodes were characterized by cyclic voltammetry and UV-Vis spectroscopy. The experimental observations indicate the degradation of the substituted-POMs during their incorporation in the PAH multilayer film, probably as a result of electrostatic repulsions between the positively-charged metal cation substitute (Fe(III) Mn(II) or V(IV)) of the POM and the positively-charged PAH. The degradation mostly results in the formation of lacunary-type POMs. Although the encapsulation does not lead to the entire preservation of the substituted-POMs, the substituted-POMs/PAH films can be used as pH sensors and as efficient electrocatalysts for the reduction of dioxygen, hydrogen peroxide and the oxidation of the NADH coenzyme. (C) 2010 Elsevier B.V. All rights reserved.