We have shown that biphenyl, the central structural block in polychlorinated biphenyl (PCB) molecules, is also a key component of PCB reaction networks in supercritical water (SCW). This study provides information on biphenyl thermolysis (SCWT) and oxidation (SCWO) in SCW, proves the high chemical stability of biphenyl to SCWT/SCWO conditions, and exposes the complexity of the opening-ring reactions involved in overall PCB reaction networks. The decomposition reactions of biphenyl are investigated at 25 MPa and 673-823 K in SCW in the presence of methanol, with and without oxygen (from H2O2/H2O solutions). The experiments with biphenyl, delivered to a tubular reactor both as solutions in methanol (2-6 g/L and 5 wt %) and neat (melted) reactant, are conducted isothermally at plug flow conditions. GC-FID/TCD/MSD chromatographic methods are employed for product analysis. Global conversions for biphenyl disappearance under SCWO conditions vary from 3% (673 K and 3.8 s) to 97% (823 K and 14.2 s) when delivered at 2-6 g[L and follow overall second-order kinetics with Arrhenius parameters A = 10(24+/-0.3) (mol/L)(-1) s(-1) and E-a = 294 +/- 5 kJ/mol. The global kinetic model developed from these data is also valid for the SCWO of biphenyl/MeOH at 5 wt %, but it fails in the case of neat biphenyl. These findings suggest an important contribution of methanol in the reaction mechanisms, products, and pathways. The main primary reaction products are all three hydroxybiphenyl isomers. However, more than 50 other minor reaction intermediates (yields less than a few tenths of a percent) have been identified under different SCWO conditions. No significant reactions are observed under SCWT conditions.