The ability of subcritical water to support ionic chemistry is evident from the successful generation of 9-R-xanthenium (R = H and C6H5) carbocations at temperatures up to 330 degreesC from their respective alcohol precursors using laser flash photolysis. The intrinsic carbocation decay in the solvent was found to follow simple Arrhenius behavior at all temperatures, from ambient up to 330 degreesC. In addition, we determined the bimolecular rate constants for the reactions of xanthenium cation and 9-phenylxanthenium cations with amylamine, a neutral nucleophile. The activation energies of the two reactions were 21.6 +/- 1.2 kJ mol(-1) and 18.2 +/- 3.9 kJ mol(-1), respectively. More importantly, we found that the rate equations determined at high temperatures extrapolated extremely well to ambient conditions. Thus, we conclude that for ion/neutral isoCoulombic reactions, it would be reasonable to use low-temperature Arrhenius parameters to predict the rate constants in water at higher temperatures. Finally, we found that the effect of pressure on the bimolecular rate constants in the temperature range of 22-330 degreesC was negligible.