[en] To be able to deduce water holdup from the mixture conductivity measured by conductance sensors in water continuous oil–gas–water three-phase flow, the water conductivity must be known in advance. As water conductivity depends on water salinity and temperature, variation in water salinity or temperature changes the water conductivity, which seriously affects water holdup measurement. In this study, we propose a novel online technique to detect the conductivity of water in vertical upward oil–gas–water flows in 20 mm inner diameter pipe via the conductance method. A water conductivity sensor (WCS) is designed and the geometry of the electrodes in its water trapping cavity is optimized using finite element methodology. After this, the relationship between sensor response and water conductivity is investigated. WCS performance is experimentally evaluated and the results show that the WCS can effectively detect the water conductivity in a three-phase mixture online. By correcting the water conductivity in the expression for the normalized conductivity of a rotating electric field conductance sensor, water salinity and temperature independent water holdup measurement can be realized. Based on the relationship between the calculated water holdup and water cut, the superficial velocity of water is accurately predicted despite differences in water conductivity. (paper)