The kinetics of formate synthesis and formate decomposition on a Cu(lll) surface has been studied using an XPS apparatus combined with a high-pressure flow reactor. The activation energy of the formats synthesis by hydrogenation of CO2 at 1 atm and a mole ratio of CO2/H-2 = 1 is determined to be 64.1 +/- 2.2. kJ mol(-1). A kinetic analysis concludes that the formation rate cannot be explained by a simple Langmuir-Hinshelwood mechanism. A new Eley-Rideal mechanism is proposed. The rate constant of the formate decomposition is very different depending on the preparation method of formate, that is, synthesis by hydrogenation of CO2 at 1 atm and adsorption of HCOOH on an oxygen-covered Cu(lll) surface in UHV. However, the activation energies of the decomposition for both cases are close to each other, meaning that the frequency factor of the rate constant is different for the two cases. This can be explained by the different surface structures of formate previously observed by STM.