An instrument is described that measures Eh and pH simultaneously in boreholes. The temperature of the borehole fluid and its self-potential with respect to an up-hole mud probe can also be measured. Operation should be possible in water depths to 100 m. By connecting the pH electrode system in a feedback circuit, electrometer amp lifier performance is obtained from a cheaper, lower quality amplifier. Signal subtraction operations allow the same reference electrode to be used simultaneously for all measurements. Sources of error are reduced by careful guarding of the high-resistance input circuits and by using isolation amplifiers to prevent ground leakage currents. A number of future development possibilities are indicated.