We have built an experimental apparatus to study chemical reactions on surfaces by spectroscopy. We currently use this appartus to study the decomposition of water on a zirconium crystal. The products of decomposition are zirconium oxyde and molecular hydrogen. The decomposition of water on metallic surface is of great interest to many areas of chemistry (catalysis, corosion and hydrogen storage). We use three photon REMPI spectroscopy on the molecular hydrogen produced by the reaction to determine its translational, rotational and vibrational energies. This information willl be use to determine the mecanism of the reaction.