A bleached sulfate pulp has been refined in a disk refiner to different degrees : from 15 to 75° SR (720 to 70 CSF) . Different water contents ranging from 40 to 90 % by volume (70-90 % by weight) , have been obtained by centrifugation of small samples of ca.30 mg o.d. weight. The samples have been analyzed by microwave spectrometry performed in a cavity at 5 GHz. The theory of small perturbations of a resonant cavity allows the determination of the dielectric losses Es of the hydrated material. The slope of the lines : Es vs. moisture content is an index of the hydration of pulps. At the same water content (in the range 70 to 90 % by weight) , we have shown that refining increases the hydration, which can be defined as the transformation from free water to bound water.