This study assesses the nature of sorbed water and the related hygrothermal effects in epoxy resins. This paper is the first of a two-paper series. Three epoxy systems, DGEBA + mPDA, TGDDM + DDS, and Fiberite 934, were used in the investigation. Water sorption was achieved by immersing the materials in distilled water at constant temperature of 45 degrees C, 60 degrees C, 75 degrees C and 90 degrees C for 1530 h. Water absorption and desorption profiles were analyzed to determine the diffusion parameters. Solid state nuclear magnetic resonance (NMR) was conducted to determine the mobility of water in epoxy. The study shows that water molecules bind with epoxy resins through hydrogen bonding. Two types of bound water were found in epoxy resins, The binding types are classified as Type I or Type II bonding, depending on difference in the bond complex and activation energy. The activation energy of Type I and Type II bound water is similar to 10 and similar to 15 kcal/mol, respectively. Type I bonding corresponds to a water molecule which forms a single hydrogen bond with the epoxy resin network. This water molecule possesses a lower activation energy and is easier to remove from the resin, Type II bonding is as a result of a water molecule forming multiple hydrogen bonds with the resin network. This water molecule, therefore, possesses a higher activation energy and is correspondingly harder to remove. Type I bound water is the dominant form of the total amount sorbed water. The amount of Type II bound water depends strongly on the exposure temperature and time. Higher immersion temperature and longer exposure time result in a greater amount of Type II bound water.