Naproxen sodium (NS) is a nonsteroidal anti-inflammatory drug used in painful and inflammatory diseases. By crystallization from water or by exposure to relative humidities over 43%, the anhydrate form can be hydrated to a dihydrate species. Different techniques have been used to characterize physically anhydrate naproxen sodium (ANS) and hydrate naproxen sodium (HNS): elemental analysis, atomic absorption, electron scanning microscopy, thermomicroscopy, differential scanning calorimetry, Karl Fisher's titrimetry, thermogravimetry, spectrophotometric analysis and X-ray diffraction study. The hydration/dehydration mechanism, at different relative humidities, was investigated to evaluate their physical stability. When stored up to 43% relative humidity, ANS shows a good stability, whereas with an increase in relative humidity it is hydrated. HNS equilibrium solubility was determined at different temperatures (21, 26, 31, and 37 degrees C). Due to the metastability and the quick phase changes in the water of ANS, its solubility was calculated from intrinsic dissolution measurements at the same temperatures, as solubility measurements of HNS. Water solubility of ANS is greater than HNS, but the solubility difference decreases when the temperature decreases. This is due to the fact that at higher temperatures the intrinsic dissolution rates (IDR) of ANS are considerably faster and decrease as the temperature falls.