We show that vibrational energy can be used to tune the chemisorption state of an adsorbed molecule. When dosing C2H4 with a supersonic molecular beam on Ag(0 0 1) we demonstrate by vibrational spectroscopy that, with increasing nozzle temperature, TN, firstly no stable adsorption occurs, then a pi -bonded configuration is populated, switching finally to a more strongly, undissociated, di-sigma -bonded state. Direct dissociation is observed at still higher T-N with the formation of dehydrogenated radicals.