The decomposition reactions of 4-chlorobiphenyl (4CB) are investigated at 25 MPa and 773 K in supercritical water (SCW) in the presence of methanol and then benzene both without and with oxygen (from H2O2/H2O solutions) to understand the co-oxidation effects on PCB conversion and reaction products. The reactions of 4CB, delivered to a tubular reactor as solutions in methanol and benzene (3 g/L), are conducted isothermally under plug-flow regime, and GC-FID/TCD/MSD chromatographic methods are employed for product analysis. At similar oxygen/organic equivalence ratios (Phi(1) = 1.2 and Phi(11) = 1.3), global conversions of this simple PCB congener vary from 79 to 97% when delivered in methanol (I) and from 61 to 88% when benzene (II) has been used as co-solvent (2-24.5 s reaction time). Under the same initial oxygen concentration (0.144 mol/L; Phi(1) = 1.2 and Phi(11) = 0.5), molar conversions of 4CB in the latter system are even lower (48-72%). These results show the important role of methanol as reaction rate enhancer compared to benzene. Moreover, the identified reaction products of 4CB/MeOH and 4CB/C6H6 differ significantly in these systems, showing different reaction pathways. A dramatic increase in the number of reaction products has been observed in the presence of benzene compared to 4CB/MeOH reactions. Further, the more toxic dibenzofuran has been produced in the largest amount in the 4CB/C6H6 system and significant oligomerization of benzene/biphenyl also occurs. Even the most toxic dibenzo-p-dioxin was detected in amounts at the level of unreacted 4CB. These results clearly show the significance of the co-oxidation effects on PCB reaction rates, pathways, and products in SCW. (C) 2004 Elsevier B.V. All rights reserved.