in many reactions catalyzed by polyoxoanions, the first step involves attaching a proton at the anion so that the activated species can react with the substrate. The present study focuses on this first step and makes a detailed analysis of the factors that affect the basicity of the distinct oxygen sites in a polyoxometalate. Several prototypical polyoxoanions were studied in solution using the conductor-like screening model (COSMO) to model the solution. The nucleophilicity of a given site depends mainly on the coordination type, the metal to which the oxygen is linked, the charge of the anion and the bonding angle in the bridging oxygen sites. The coordinated metal and the coordination type are the most important. We discuss in detail the effect of the charge and of the charge density of the anion on the basicity of the polyoxoanion. (c) 2006 Elsevier B.V. All rights reserved.