A hygrometric method is presented with which the sorption isotherm of water in paint films can be measured. Only standard laboratory equipment is used. The principle of the technique is that a known amount of water is added to a system of paint and air. The relative humidity in the air is measured. The amount of water in the paint follows from a mass balance. A small leakage is incorporated in the model. The method is used on three alkyd paints and three latex paints. (C) 2003 Elsevier B.V. All rights reserved.