A comparative kinetic study of the rat-liver glutathione S-transferase catalysed dehalogenation of fluoro- and chloroaromatic compounds showed no significant decrease of the F/Cl mobility ratio on going from substrates that carry an ortho-nitro group to those which lack nitro groups near the reaction centre. This suggests a so far unrecognized activity of S-transferases in providing "solvation" for the leaving halide anion at the hydrophobic reaction centre. (C) 1998 Elsevier Science Ltd. All rights reserved.