Phase behavior and reaction of polyethylene (PE) in supercritical water were studied with a diamond anvil cell (DAC) technique with visual and Raman spectroscopy. When PE + water (12-30% PE) mixtures were rapidly heated at initial pressures ranging from 110 to 690 MPa, PE first melted and formed a liquid spherule PE phase. The spherule began to expand at above 450 degrees C and underwent a color change to red at about 570 degrees C. At higher temperatures, the red color disappeared and the PE molten phase turned transparent. Upon further heating, the red color returned and other material underwent homogenous reaction as evidenced by a dark color which appeared throughout the cell. Volatile liquids were formed on the surface of the liquid PE phase spherule. For reactions run at higher temperature (645-671 degrees C) at pressures ranging from 1.9 to 2.6 GPa, thin films formed on the anvils after quenching which had C=C, OH, and C-C Raman bands, which indicated that hydrolysis products formed even though the reaction times were relatively short (290-475 s). Reactions performed at a constant temperature of 423 degrees C and at an initial pressure of 850 MPa showed only a slight decrease (0.03 MPa/s) in pressure with time. The results of this study show, conclusively, that PE and water remain as a heterogeneous system over the polymer (12-30% PE) compositions studied during heating and reaction in supercritical water. Only after PE decomposes to lower molecular weight hydrocarbons, above about 565 degrees C, can homogeneous reaction conditions result. (C) 2000 Elsevier Science B.V. All rights reserved.