pH microsensors were incorporated into a microcapillary cell to monitor pH changes simultaneously with electrochemical measurements. The microsensors (diameter d = 10 mu m) were positioned at the center of the microcapillary cell tip opening (d = 100 mu m) and 50 mm above the surface of the sample. During the dissolution of MnS inclusions on AISI 304 stainless steel the pH of the nearby electrolyte decreased to a value close to 4 in 1 M Na2SO4. In 0.1 M NaCl, pitting corrosion occurred at MnS inclusions, and the metastable and stable pitting events acidified the local environment. The metastable pits decreased the pH of the electrolyte to values between 2 and 4. The stable pits caused the pH of the electrolyte to decrease to similar to 2.