This paper presents an in situ, nondestructive method of monitoring Cl- concentrations and pH values at the steel/concrete interface. The Ag/AgCl electrodes prepared by the electrochemical anodization and the Ir/IrO2 electrodes prepared by thermal oxidation in carbonate served as Cl- concentration and pH sensors, respectively. The potentiometric response of the Ag/AgCl electrode to the logarithm of Cl- concentrations ranging from 1 x 10(-4) to 2 M in saturated Ca(OH)2 solution simulating the inner electrolytic medium of concrete shows good linearity. The Ir/IrO2 electrode also exhibits an ideal Nernstian response in the range of pH 1-14. The Ag/AgCl and Ir/IrO2 electrodes were combined into a multiplex Cl-/pH sensor, and the sensor was embedded in concrete close to the steel/concrete interface to realize an in situ and long-term measurement of Cl- concentrations and pH values. The results indicate that the combined sensor is robust and sensitive enough to in situ measure Cl- concentrations and pH quantitatively at the steel/concrete interface, which is of indispensable importance to the study of corrosion and protection of the steel in concrete.