Small changes of local pH in the vicinity of an electrode surface are observed and quantified for the first time using the infrared attenuated total reflection technique (ATR). The local pH changes occur during hydrogen evolution close to the ATR prism used as the cathode in an electrochemical cell containing a phosphate buffered electrolyte. pH changes are deduced directly from the phosphate concentration changes. Under optimum conditions, a change as small as 10(-4) pH unit can be measured. A linear approximation model of the transport in the cell accounts very well for the dependence of pH changes on the electrochemical current density, but fails to give an absolute prediction as accurate as the experimental determination. These observations offer new possibilities of measuring small changes of local pH in a wide pH range using different acid-base pairs.