Dilute solutions of NaCl in supercritical water (SCW), in concentrations up From 0 to 3.1 wt.% have unexpected influence on the rate of hydrolysis of diphenylether (DPE). The rate at 430 degrees C and at a SCW density of 0.46 g/cc showed a sharp decrease with incremental addition of salt, down from the value measured at zero salt. But the rate increased again with continued addition of salt and reached a value almost twice that of the zero-salt rate when the salt concentration was 3.1 wt.%. The decrease of the rate is attributed to the formation of the ion pairs H+ Cl-; the excess of Cl- ions that prevail from ionic (partial) dissociation of NaCl capture the protons generated by self-dissociation of water, according calculations of the relevant ionic dissociation equilibria. The formation of protonated DPE, as a critical intermediate species in the chemistry of DPE hydrolysis according the postulated S(N)1 mechanism, competes with this ion pair for protons and is consequently restrained. At the higher salt content a new mechanism predominates. This mechanism is rationalised with Lewis acid/base behaviour of Na+ and Cl- ions in SCW solution. Charge transfer in the water cluster surrounding these ions supposedly generates H+ and OH- ions at the outer hydration shells of the clusters and form the active species for DPE hydrolysis. The rate of hydrolysis, according this Lewis acid/base theory is proportional to the square root of the salt concentration in the SCW reaction medium and this was indeed corroborated by experiments. (C) 2000 Elsevier Science B.V. All rights reserved.