An in-situ calibration method to accurately and precisely constrain the pH of deep sea environments is reported. This method involves the pre-evaluation of the Ir-IrOx|Ag-AgCl electrode in the laboratory, followed by the in situ calibration using pH buffers at the ocean bottom. The standard potential E° and in situ slope are derived by the calibration process. This calibration method was implemented in an in situ calibration device (pH Calibrator) and used in the seawater column and hydrothermal diffuse flow measurements. The precision, accuracy and stability of the pH value determined are sufficient for many geochemical processes.