Reaction rates were measured for the reaction of the hydrated electron with sodium nitrate. sodium nitrite. and perchloric acid using pulse radiolysis/transient absorption at temperatures Lip to 380 degreesC. For nitrate and nitrite, the rates increase as the temperature increases. reach a maximum around 125-200 degreesC and then decrease. Above 350 degreesC the rates increase again sharply. The perchloric acid rates strongly increase between 250 and 350 degreesC, and decrease sharply at 380 degreesC. We interpret the unusual rate behavior at supercritical temperatures in terms of ion pairing, which eliminates coulombic attraction/repulsion between the reactants. (C) 2002 Published by ElSevier Science B.V.