This paper presents new data on the behavior of benzo(a)pyrene (BaP) during supercritical water (SCW) combustion. It focuses on phase changes of the BaP throughout the transition from subcritical to supercritical regions. A sequence of images illustrates BaP's phase change for the first time. They were obtained in situ using a hydrothermal diamond anvil cell coupled with Fourier transform infrared spectrometer as well as optical and infrared microscopes. Combustion/reaction at different oxygen concentration (0-49% H2O2), and pyrolysis experiments were conducted. The results show conclusively that (1) BaP is stable at pyrolytic conditions up to 452 degreesC; (2) It can disserve in supercritical water at 442-452 degreesC forming partly decomposed globule. At extended reaction times above 500 degreesC the globule undergoes carbonization while the dissolved compounds inhibit char formation. No complete dissolution was observed. (3) BaP combustion occurs simultaneously with dissolution in a single homogenous phase. At higher oxygen content, the dissolution and complete combustion takes place even in subcritical region (353 degrees and 145 MPa). In such a case no melting phase is present. These observations may affect the design and organization of the SCW combustion process. (C) 2001 by The Combustion Institute.