The water-gas shift (WGS) reaction \( ({\text{CO}} + {\text{H}}_{ 2} {\text{O}} \to {\text{CO}}_{ 2} + {\text{H}}_{ 2} ) \) is a key process to the production of high purity H2 from gas streams rich in CO. The identification of the WGS reaction mechanism and the probable stable intermediates is critical to design the catalyst structure, optimize composition and tune reaction kinetics/thermodynamics to achieve the optimum selectivity and activity. In this study, first the WGS reaction steps on Cu(111) have been studied using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy under ultra-high vacuum (UHV) conditions. Then the interactions of H2O with CO on Cu(111) have been studied under elevated pressures (90 mTorr CO + 30 mTorr H2O) at 300-575 K with ambient pressure XPS. Under UHV conditions, non-dissociative adsorption of H2O on Cu(111) and Cu2O/Cu(111) was observed. Whereas H2O readily dissociates, by breaking the O-H bond on a chemisorbed O layer on Cu(111) to form OH species. Even though this OH interacts with adsorbed CO, it does not react to form any associative intermediate and simply desorbs as H2O at 275 K under UHV conditions. At ambient pressures, no associative intermediates species, only CO and OH, were observed in the reaction of CO with H2O although the catalytic production of H2 can be detected under these conditions. Since intermediate species other than CO and OH were not observed on Cu(111) under reaction conditions, we concluded that the redox mechanism is the dominant WGS pathway on Cu(111). The coupling of Cu to an oxide, Cu-CeO2 catalyst, or a carbide, Cu-TiC catalyst, favors an associative mechanism and produces a very large increase in the rate for the production of H2 through the WGS.