The test method describes the procedure for the direct determination of water concentration in polyol ester and diester based aerospace lubricants by the commercially available automated coulometric Karl Fischer titration instrument. The method was validated to cover the water concentration range of 150 to 3500 µg/g. The method may also be suitable for the determination of water concentrations outside this range and for other classes of fluids, however, the precision statement shall not be applicable for such uses.