Water is ubiquitous in many catalytic reactions used in the upgrading of biomass. Therefore, quantifying and controlling the influence of water in activity, selectivity, and catalyst deactivation is essential for advancing this technology. Here, we report a kinetic study of the cyclopentanone self-condensation over functionalized MCM-41 catalysts with varying acid densities and in the presence of controlled amounts of water. It is observed that small amounts of water enhance the activity of these catalysts. A detailed analysis of the experimental data indicates that the rate limiting step is the enol formed via protonation and tautomerization of a cyclopentanone molecule attacks the electrophilic carbon of a second carbonyl. On catalyst with high density of sulfonic acid groups, the best fitting of the experimental data is obtained with a kinetic model in which the C-C bond forming step mostly occurs with the second carbonyl molecule adsorbed (and polarized) by an adjacent acid site (Langmuir-Hinshelwood, L-H). By contrast, at a lower density of sulfonic acid groups, the number of active sites with enough proximity greatly decreases and the C-C bond forming occurs with the second carbonyl in the liquid phase (Eley-Rideal, E-R). In this case, it is observed that when water is added to the reaction mixture, the best description of the kinetics switches from E-R to L-H. Evidently, the water can help bridging active sites remotely located on the surface, extending the polarizing effect of the second acid site. (C) 2019 Elsevier Inc. All rights reserved.