Infrared spectral measurement was carried out to find the crystallization of the amorphous poly(ethylene imine) (PEI) sample into the hydrates by absorbing water molecules. When PEI sample was melted and cooled to room temperature in dry state, it was found to crystallize into the anhydrate consisting of doubly stranded helices. But when the sample was melted and subjected to the humid atmosphere at the same temperature, the formation of hydrates was found to occur. The hydrates were considered to have higher melting points than the anhydrate, in other words, they were more stable than the anhydrate in higher temperature region. (C) 2003 Elsevier Science Ltd. All rights reserved.