The water uptake of three polyurethane adhesives was studied by an in situ fluorescence method applying extrinsic fluorescent probes, The measurements were made in water immersion at 50 and 80 degreesC, and in water vapor at 50 degreesC. An intensity ratio method was applied, in which ratios of the highest and lowest intensity changes in the fluorescence emission bands were used to determine the amount of water sorbed in the polymer films. The ratio method was used to compare the fluorescence data with those obtained by a conventional gravimetric method. Comparable results were obtained.