We determined chloride and sulfate concentrations and δDH2O and δ18OH2O isotope ratios of interstitial fluids extracted from cores from two fault zones within the Taiwan Chelungpu fault. The fluids had higher concentrations of chloride and sulfate and higher δDH2O and δ18OH2O values than the local meteoric water. We concluded that the higher δDH2O and δ18OH2O values were the result of either entry of surface meteoric water at low altitude via fault zones, or isotopic fractionation by depletion at 50°C in a shallower fault zone, and at 125°C in the deeper fault zone. The high temperature in the deeper fault zone might have been earthquake-related frictional heating, which led to isotopic fractionation. However, neither of the above hypotheses explains the high concentrations of Cl- and SO42-. We concluded that the high concentrations of Cl- and SO42- might have resulted from mixing with seawater and dissolution of pyrite, respectively.