Relaxation measurements on CO and HCl excited into the first vibrational state and on CO(2) and N(2)O excited into the (00 degrees 1) vibrational state have been performed by using a fluorescence method. The rate constants associated with the de-excitation by molecular collisions have been determined at room temperature. The diffusion and accommodation coefficients have been deduced from measurements at low gas pressures, taking into account the self-absorption of the fluorescence radiation.