The water content of solutions is important for fuels, lubricants, hydraulic liquids, oils and fats, and enzymatic catalysis in organic liquids. Rapid and accurate on-line water detection is needed for research and for the technical realization including process optimization of enzymatic catalysis in organic liquids. This paper reports on the development of sensors that rely on the selective pervaporation of water from micro-aqueous organic liquids to the vapor phase through a polymeric membrane. This sensing principle employs the transport of water exploiting both diffusion and sorption. The water content of liquid n-hexane and kerosene is sensed from saturation down to concentrations in the low ppm range. The isothermal sensor response is shown to be largely linear with dissolved water concentration. (C) 2004 Elsevier B.V. All rights reserved.