The formation of water from atomic oxygen and atomic hydrogen coadsorbed on the Pt(111) surface has been characterized by x-ray photoemission (XPS) and temperature programmed reaction (TPR). These studies were performed between 100 and 155 K, which is below the desorption temperature for adsorbed water on the Pt(111) surface. Only adsorbed atomic oxygen and atomic hydrogen were present on the surface prior to reaction. At 100 K no reaction occurs; however, at 120 K the reaction goes to completion even in the absence of gas phase hydrogen. Therefore, water formation on the Pt(111) surface has a low activation energy with an upper limit of 8 kcal/mol (34 kJ/mol). Although previous work has shown that hydroxyl groups are stable near 150 K on the Pt(111) surface, no significant surface concentration of OH species was detected during these reaction studies. The reaction mechanism most consistent with all of the data is the sequential addition of atomic hydrogen to oxygen and then to hydroxyl groups to form the water product. A low surface concentration of OH groups is expected within this mechanism if OH formation is the rate-limiting reaction step.