The effects of a sodium hydroxide (NaOH) addition on the decomposition of phenol and 2-chlorophenol (2CP) were investigated in a flow reactor, in the temperature range of 573-733 K and at a pressure of 26 MPa. The reactor residence times ranged from 2.7 to 176.6 s. The initial phenol and 2CP concentrations were 1.06 x 10(-2) and 7.78 x 10(-3) Mol/L, respectively. The initial NaOH concentrations were 0 and 500% of the molar concentration of phenol, and they ranged from 0 to 700% of the molar concentration of 2CP. Under the experimental conditions, the conversions of phenol were below 5% regardless of the NaOH addition. On the other hand, the conversions of 2CP were between 1.0 and 99.9%. The addition of NaOH accelerated the decomposition of 2CP in supercritical water (SCW), but it did not influence the decomposition of 2CP in subcritical water. GC/MS analysis of the 2CP effluent identified intermediates such as 2-cyclopenten-1-one, phenol, cresols, acetone, and acetaldehyde at a residence time of 6.7 s. In the gas samples, CH4 and H-2 were most abundant, but some CO2 was detected. Chlorinated compounds and dimers were not detected in the liquid samples, which shows that the NaOH addition in SCW promotes the dechlorination of 2CP and the decomposition of dimers.