We present a two-state model for CO electrooxidation on catalyst nanoparticles. It exploits the active site concepts in order to describe the effects of the heterogeneous surface structure on the catalytic activity of nanoparticle systems. In this approach, the apparent reactivity results from the interplay between kinetic processes that occur on active sites and surface transport of adsorbed reactants from inactive towards active sites. It is demonstrated that this model is a generalization of well-known mean field (NIF) and nucleation and growth models. Kinetic Moute Carlo (kMC) simulations specifically developed for this problem were employed for establishing the relevance of the different model parameters for the shape of chronoamperometric and linear sweep current transients. In the limit of fast COad surface mobility, the corresponding NIF approximation with active sites and its analytical solutions for limiting cases are presented. The comparison of the general kMC solution with the MF approximation reveals major applicability limits of the NIF approach for heterogeneous surface models. Although the current work concentrates on the specific case Of COad adlayer electrooxidation, the model is readily applicable for other reactions where surface heterogeneity is likely to play an important role. (c) 2007 Elsevier B.V. All rights reserved.