Phase behavior and liquefaction of styrene-butadiene rubber (SBR) in supercritical water (SCW) were studied with a diamond anvil cell (DAC) technique coupled with optical microscopy and FT-IR spectroscopy. Apparent concentrations were calculated using digital imaging analysis. When SBR + H2O + H2O2 systems (15.0-28.8 wt% SBR) were rapidly heated at a rate of 2.7-9.7degreesC s(-1) at pressures ranging from 809 to 1038 MPa, SBR particle began dissolving at 542-546, 196 and 201degreesC with 0, 5, and 10 wt% H2O2 concentration, respectively. Solubility increased with H2O2 concentration. After solubility reached the maximum at 521-558degreesC, a non-dissolved particle expanded and changed to reddish volatile compounds at 535-585degreesC, which underwent liquefaction and then carbonization as temperature increased to 686degreesC. The dissolved compounds in water, inhibited formation of char. For the isothermal runs at 450degreesC and 395-721 MPa, liquefaction started at 1628, 663, and 53 s with 0, 5, and 10 wt% H2O2 concentration, respectively. The results show conclusively that the SBR can dissolve in SCW while non-dissolved residue undergoes liquefaction. Addition of H2O2 promoted the liquefaction process. (C) 2002 Elsevier Science Ltd. All rights reserved.