The thermal behavior of poly(ethylene imine) (PEI) hydrates in a water vapor atmosphere was investigated through temperature-dependent measurements of infrared spectra and X-ray diffraction. Almost perfectly dried anhydrate melted at about 60 degreesC during the heating process. Anhydrate containing a small amount of water showed a phase transition to a mixture of hemihydrate and sesquihydrate around 40 degreesC, at which point the ethylene imine (EI)/water ratio was 1/0.5 in the hemihydrate and 1/1.5 in the sesquihydrate. The hemihydrate transferred to the sesquihydrate around 60 degreesC, and the latter melted above 80 degreesC. When the starting PEI sample contained a greater amount of water and consisted of hemihydrate and sesquihydrate, the hemihydrate transferred to the sesquihydrate via heating, and the latter melted around 75 degreesC. For a sample of dihydrate (EI/water ratio = 1/2) containing an appreciably large amount of water, it transferred to the sesquihydrate around 65 degreesC, and the latter melted above 90 degreesC. A sample of dihydrate with a much higher water content existed up to 110 degreesC and then melted; during this period, no transition to the sesquihydrate was observed. In this way, the starting crystalline phases were found to change for anhydrate and various types of hydrates. Their transition behaviors varied according to the water content. From these data, a phase diagram was successfully derived as a function of the temperature and water content. This phase diagram allowed us to predict the transition behavior during the hydration process at various constant temperatures. For example, at 60 degreesC, a molten sample should crystallize into a mixture of hemihydrate and sesquihydrate at first, and the hemihydrate should transfer to the sesquihydrate with increasing water content. The latter should change to the dihydrate in the final stage. This prediction was checked with time-resolved measurements of X-ray diffraction and infrared spectra during the hydration process at the corresponding temperature; this led to the establishment of the phase diagram. (C) 2003 Wiley Periodicals, Inc.