A diblock copolymer of poly(N-isopropylacrylamide) and poly(ethylene oxide) (PiPA-b-PEO) has been prepared by radical polymerization with a ceric ion initiation system. Its thermosensitive micellization has been investigated by means of IR and fluorescence spectroscopy. The PiPA segments are critically dehydrated above 33.5 degreesC (5 wt.-%) and associate through hydrophobic interaction to form the hydrophobic core of the micelle. In contrast, the change in the hydration state of the PEO segments upon micellization is small.