Using a competitive spin-trapping method, we determined the effect of high static pressure on the trapping rate of hydroxyl and phenyl radical in solution. Under various pressures, hydroxyl or phenyl radicals were produced in solution in the presence of two different spin-trapping compounds, and the yield of trapped products was used to calculate relative trapping rates. We found that external pressure increases the phenyl-radical trapping rates by approximately 40% at the pressure of 490 bar, while there was no pressure effect in hydroxyl-radical trapping rates. We interpreted the pressure effect based on the reactivity difference of phenyl and hydroxyl radicals to the spin trap which is mainly caused by the variation in the volume of the group. (C) 2004 Elsevier B.V. All rights reserved.