(WO3)(3) clusters deposited on the (110) rutile TiO2 surface are excellent catalysts for the formaldehyde (CH2O) polymerization reaction (J. Phys. Chem. C 2010, 114, 17017). The present B3LYP study unravels the possible paths of this catalyzed reaction. According to the stoichiometry of the r-TiO2 surface, the (WO3)(3) clusters can be neutral, singly charged, or doubly charged. We find that only neutral (WO3)(3) and anionic (WO3)(3)(-) clusters are reactive toward CH2O molecules. In both cases it is possible to determine more than one mechanism on the basis of a nucleophilic attack of the formaldehyde O atom to the W ions of the cluster. The reaction proceeds through successive attacks of other CH2O molecules and the formation of acetal and polyacetal intermediates, which inhibits the chain propagation. Only in the case of the anionic (WO3)(3)(-) catalyst is a totally different reaction path possible at low temperatures. This path involves formation of radical species where the unpaired electron is localized on the organic moiety bound to the cluster. The polymer chain propagation follows a radical mechanism with low activation barriers. Thus, a cluster's electron charging speeds up the formaldehyde polymerization at low temperatures. On the basis of these unexpected results, we conclude that electron-rich supports and low working temperatures are the keys to kinetic control of the reaction favoring a fast radical chain propagation mechanism.