A nonintrusive technique for the in-situ measurement of interfacial pH and current efficiency has been developed. A ring electrode, operated potentiometrically at opencircuit, is used to measure the pH change at a rotating disk electrode. The technique takes advantage of the well-characterized hydrodynamics at the rotating disk electrode and has the added advantage that the pH probe, the ring electrode, is not interfering with the flow field and the current distribution on the disk. To determine the pH at the disk electrode by measuring the potential of the ring, the radial transport of hydronium ions across the insulating gap and on the ring is analyzed taking into account the effect of homogeneous dissociation reactions of water and metal-hydroxide ion complexes. Shifts in the ring potential caused by hydrogen super-saturation and ohmic drop are also evaluated. A platinized ring electrode in a hydrogensaturated electrolyte is shown to provide a stable and reproducible hydrogen ion sensor with a Nernstian response to the changes in the bulk pH. Performance of the ring is evaluated by generating hydrogen at the disk electrode from a dilute acid solution, in the absence of other electrochemical reactions. The technique is then applied to determine the interfacial pH of nickel, iron and nickel-iron alloy electrodeposition with concurrent hydrogen evolution. This method was also used to measure the current efficiency of nickel electrodeposition in a fast, nonintrusive and in-situ manner.