The chain conformation and hydration structure of all aqueous solution of linear poly(ethylene imine) (LPEI) have been studied by the use of mid-infrared (mid-IR) and near-infrared (NIR) spectrometries to explore the molecular mechanism of an upper critical solution temperature (UCST)-type solubility change. When LPEI was dissolved in water, the solution became a gel at room temperature. Mid-IR and NIR absorption bands of the LPEI gel appeared at the same positions as the bands of the dihydrate crystal of LPEI in a film, which suggested that LPEI formed dihydrate crystallite in water at room temperature. This means that LPEI is ill a planar-zigzag structure even in water at room temperature. When the gel was heated, it was changed to a transparent aqueous Solution. Both mid-IR and NIR bands of all LPEI aqueous solution above 64 degrees C exhibited a broader bandwidth than those at room temperature. Judging from the change of the bandwidth, the LPEI chain in water was disordered by heating to take a random-coil form. In addition, band locations of the CH stretching first-overtone vibration bands ill a NIR spectrum of the Solution above 64 degrees C were higher than those of melted dry LPEI, which was reasonably understandable that the disordered LPEI chains ill the aqueous solution were ill a highly hydrated state. These spectral results have revealed that the change from the planar-zigzag form to the random-coil one in water is a trigger to begin the UCST-like solubility change oil heating followed by hydration.