Thermal energy helium scattering is used to measure the isothermal desorption of NO from Pt(111) in the temperature range 343-383 K. Taking into account the vibrational degrees of freedom to calculate the residence time of the molecules, the energy of adsorption is E/sub a/=-29.8+or-1 kcal/mol, and a repulsive interaction energy between molecules is found: phi =6.1+or-1 kcal/mol. Also, the macroscopic diffusion of NO on Pt(111) is measured for the first time between 313 and 358 K. The technique uses a helium beam to monitor the time dependence of the coverage on a well defined surface spot, when the adsorbed molecules diffuse outside this spot. The activation energy for diffusion at low coverage is E/sub D/=11.8+or-1.2 kcal/mol, and the pre-exponential factor is D/sub 0/=2.4*10/sup -3/ m/sup 2//s.