We present a study on the catalytic oxidation of the organic dye morin by hydrogen peroxide in the presence of manganese oxide nanoparticles in aqueous solution. The ultrathin manganese oxide nanoparticles consist of c(star)-disordered potassium birnessite and are immobilized on spherical polyelectrolyte brushes. The catalytic activity of these composite particles was investigated using the oxidation of morin by hydrogen peroxide as a model reaction. The oxidative degradation of morin was followed by UV/vis spectroscopy leading to an apparent rate constant k(app). We propose a modeling of the results in terms of a Langmuir-Hinshelwood model. k(app) can be related to the kinetic constant k and to the apparent adsorption constants of H2O2 and morin. Based on this model, the dependence of k(app) on temperature can be traced back to the activation energy of the rate constant k and the adsorption enthalpies of both educts on the surface of the nanoparticles. (C) 2012 Elsevier Inc. All rights reserved.