RECENT work by Reeves et al.1 has shown that, under certain conditions, electronically excited molecules of O2, N2, NO, etc., can be formed by the heterogeneous recombination of free atoms on a metal surface. In these experiments light emission was observed from the gas just above the metal surface, showing that the molecules formed were desorbed in an excited electronic state. This evidence, and data on the kinetics of heterogeneous recombination2,3, suggest that this desorption process is very rapid and that the molecules liberated may have excess kinetic energy. In this communication we report on an unsuccessful attempt to obtain evidence for such excess kinetic energy, by comparing the pressure exerted by a mixture of atoms and molecules on a surface where recombination takes place with the pressure on a surface where recombination is negligible.