The cathodic reduction reaction kinetics of free chlorine and oxygen on UNS C11000 copper microelectrodes were investigated in Edwards synthetic drinking water. OCl- increases cathodic reaction rates and thus raises open-circuit potential (OCP) toward pitting potentials. An increase in both the mass-transport factor K (where i(lim)=K omega(1/2)) for chlorine reduction (HOCl and OCl-) and OCP was observed as pH was increased from 8.5 to 9.5, and free chlorine levels were raised. Natural pitting was investigated using coupled multielectrode copper (UNS C11000) arrays exposed under the same conditions from pH 6 to 10 with various residual free chlorine concentrations (0-5 ppm). An empirical equation that forecasts the OCP as a function of pH and Cl-2 concentration was developed. This enabled an assessment of the pitting susceptibility of various waters based on the comparison of OCPs to critical pitting potentials. Pits formed when the OCP rose above repassivation potential E-rp and stopped growing once the OCP dropped below E-rp. Pitting severity, as determined by calculated pitting factors, increased with free chlorine concentration and was highest at pH 9.