We measured the kinetic energy of Cs+ and Cs(H2O)(n)(+) (n = 1-3) cluster ions formed in reactive scattering of low-energy (20-80 eV) Cs+ from physisorbed water on Pt(111). Reflected Cs+ primaries show two components in the energy spectrum: one scattered from surface water molecules appearing at low energy and another from bare Pt atoms at a higher energy. The former is efficiently converted to Cs(H2O)(n)(+), the reaction probability being close to unity at full coverage with water. The kinetic energies of the clusters are presented as a function of incident beam energy, scattering angle, and water exposure. On the basis of these results, we propose a cluster formation mechanism in which slow Cs+ picks up water molecules via an ion-surface abstraction reaction. At high energy, Cs+ penetrates into a physisorbed layer and the clustering reactions may also take place inside the collisionally activated volume of the layer.