The coordination of substituted pyridines to MTO (methyltrioxorhenium) is governed by both electronic and steric effects. For example, the binding constant of pyridine to MTO is 200 L mol(-1), whereas that of the better donor 4-picoline is 730 L mol(-1) and that of the sterically encumbered 2,6-di-tert-butyl-4-methylpyridine is <1 L mol(-1). A Hammett reaction constant rho = -2.6, derived from meta- and para-substituted pyridines, applies to this equilibrium. Pyridine stabilizes the MTO/H2O2 system and accelerates the epoxidation of alpha-methylstyrene. The steady-state concentration of MTO is decreased during the catalytic epoxidation reaction by coordinating a pyridine derivative, thus stabilizing the MTO/H2O2 system against irreversible decomposition. Pyridine as a Lewis base accelerates the generation of the peroxorhenium catalysts, whereas coordination of pyridine to the diperoxorhenium complex appears responsible for the acceleration of epoxidation. Ultimately, however, it is the Bronsted basicity of pyridine that lowers the activity of hydronium ion, reducing the rate of epoxide ring opening.