In the biological fixation of halide ions, several enzymes have been found to catalyze alkyl transfer from S-adenosylmethionine to halide ions. It proves possible to measure the rates of reaction of the trimethylsulfonium ion with I-, Br-, Cl-, F-, HO-, and H2O in water at elevated temperatures. Comparison of the resulting second-order rate constants, extrapolated to 25 C, with the values of k(cat)/K-m reported for fluorinase and chlorinase indicates that these enzymes enhance the rates of alkyl halide formation by factors of 2 x 10(15)- and 1 x 10(17)-fold, respectively. These rate enhancements, achieved without the assistance of cofactors, metal ions, or general acid base catalysis, are the largest that have been reported for an enzyme that acts on two substrates.