Supersonic molecular beam techniques have been used to investigate the reaction between hydrogen and a (2*1) oxygen overlayer on Cu(110). The reaction has been studied as a function of surface temperature T_s and incident beam translational energy. The rate of water production is undetectably low for a room temperature H₂ beam (E_{perpendicular to}=69 meV), and remains so until the normal component of the beam energy is increased to E_{4 perpendicular to} approximately 200 meV. For E_{perpendicular to}>200 meV one observes H₂O production with kinetics which reflect a Langmuir-Hinshelwood reaction mechanism, and the surface reaction continues to completion. The results suggest that the dissociative barrier accessed with translational energy can be rate determining, and the barrier itself is insensitive to oxygen coverage. Once this barrier is overcome, the surface reaction exhibits a T_s dependence and an activation energy which varies with oxygen coverage in the range 0-60 kJ mol^-1.