Supercritical and subcritical water oxidation of dissolved phenol and hexanol were carried out at temperatures of 315-426 degrees C under a pressure of 24.2 MPa in a tubular reactor. The effects of slow and fast preheating of the feed wastewater, and the concentration of dissolved phenol on the oxidation efficiencies, were studied. The results showed that with slow preheating, the destruction efficiencies were significantly lower than with fast preheating for both phenol and hexanol oxidation. Phenol was also observed to be more resistant to oxidation at supercritical water reaction conditions than hexanol, Higher concentrations of phenol in the wastewater sample results in higher destruction efficiencies, thus indicating variable oxidation kinetics which is concentration dependent.