We have studied reactive ion scattering (RIS) of hyperthermal (1-100 eV) Cs+ projectiles from physisorbed surfaces. RIS experiments from physisorbed water on Pt(1 1 1) reveal scattering products of Cs(H2O)(n)(+), (n = 1-3) cluster ions. The yields for RIS products are extremely high compared to those with chemisorbed species. Classical molecular dynamics simulations provide a new mechanism that explains the enhanced RIS yields with physisorbed species. Slow Cs+ projectiles pick up physisorbed molecules via an ion-surface abstraction reaction, preferably without direct collisions between projectile and adsorbate. This RIS process is very efficient and mechanistically different from the RIS process responsible for chemisorbed species that occurs through direct collision-induced desorption. (C) 2002 Elsevier Science B.V. All rights reserved.