The compound MeRe(S)(mtp)(PPh3), 2, where mtpH(2) is 2-(mercaptomethyl)thiophenol, was used to catalyze the reaction between pyridine N-oxides, PyO, and triphenylphosphine. The rate law is -d[PyO]/dt = k(c)'[2].[PyO](1/2), with k(c)' at 25.0 degreesC in benzene = 0.68 (4-picoline N-oxide) and 3.5 x 10(-3) dm(3/2) mol(-1/2) s(-1) (4-NO2-pyridine N-oxide). A chain mechanism with three steady-state thiorhenium species as chain carriers is implicated.