A quantitative thermodynamic approach is considered with the aim to describe the size-dependent Langmuir-Hinshelwood mechanism and the two-step catalytic cycle. The general treatment takes into account surface energy excess due to an intrinsic increase in chemical potential with size decrease as well as the changes in chemical potential upon adsorption. Numerical simulations as well as quantitative analysis show that for catalytic reactions over nanoparticles not only the rates but also reaction orders can vary depending on the size of nanoclusters. Comparison with experimental data is given (C) 2009 Elsevier B.V. All rights reserved.