The complexation of ultrahigh-molecular-weight poly(ethylene oxide) (PEG) with urea in both the solid and molten states was studied by infrared spectroscopy. Information about the temperature dependences of the absorption bands in the 3600-2600 and 1300-600 cm(-1) region on heating and cooling was obtained. Some conclusions concerning the interaction between PEO and urea were also made. The formation of a high temperature, metastable molecular complex between PEO and urea, which is susceptible to undercooling, was confirmed spectroscopically. Heating to 85-90 degrees C resulted in a molecular complex stable at room temperature.