We report molecular dynamics calculations and molecular beam experiments on trapping and desorption as a function of surface temperature and initial gas conditions for the Ar-Pt(111) system. The trapping process involves very rapid equilibration of the normal component of incidence velocity but extremely slow accommodation of the parallel component. At high surface temperatures for which the residence time of Ar is sufficiently short (e.g., roughly 40 ps at 273 K), trapped atoms desorb before their incident parallel velocity is thermalized. Thus trapping in the usual sense of complete equilibration with the surface does not occur; instead these quasitrapped atoms are characterized by full accommodation of only the normal velocity component. In both simulations and experiments there is a range of temperatures for which the desorbing flux associated with quasitrapping is distinguishable from the flux due to direct inelastic scattering. These results are qualitatively reproduced by a simple model for the time-varying velocity distribution of quasitrapped atoms, which treats the different components of velocity independently. © 1991 American Institute of Physics.