The effects of the addition of NaOH on the decomposition of 2-chlorophenol (2CP) and phenol in supercritical water (SCW) and under supercritical water oxidation (SCWO) conditions were investigated. The experiments were conducted in a plug-flow reactor at 713 K and 26 MPa. The reactor residence times ranged from 0.13 to 0.51 s. The initial concentrations of 2CP and phenol were 3.89 x 10(-3) and 5.3 x 10(-3) mol/l, respectively. Experiments were conducted in the presence and absence of H2O2 or NaOH in SCW. Under the experimental; conditions, the addition of NaOH accelerated the decomposition of 2CP in SCW, but had little effect on that of phenol. On the other hand, the addition of NaOH accelerated the decomposition of both compounds under SCWO conditions. From 2CP and phenol SCWO, many dimers were identified with a GC/MS. The addition of NaOH to 2CP and phenol SCWO reduced the generation of these dirners, and promoted the dechlorination of 2CP in SCW and under SCWO conditions. These results show that the effects of NaOH on the decomposition of 2CP and phenol SCWO are not negligible, and the effects of NaOH on the decomposition of other organic compounds under SCWO conditions should be considered for determining optimum operating conditions and reactor designs. (C) 2002 Elsevier Science B.V. All rights reserved.