The competition between direct-inelastic and trapping-desorption scattering of NO from Ir(111) has been studied using multiphoton ionization and time-of-flight mass spectrometry. Molecules interacting by each mechanism were observed and characterized by their angular, velocity, and internal state distributions. For Ts<300 K both channels are observed, while for Ts>300 K only trapping-desorption is observed. At T s<300 K, the trapping fraction was ≃0.85. The shift in scattering mechanism appears to be caused by a change in the surface composition due to dissociation of chemisorbed NO near room temperature. © 1988 American Institute of Physics.