The reduction of pyridine N-oxides by 1-hydroxy-1-methylethyl radicals,(.)CMe(2)OH, is acid-dependent, in accord with the greater reducibility of protonated pyridine N-oxides. The respective rate constants for the unprotonated (k(RO)) and protonated (k(R1)) pyridine N-oxides have the following values (L mol(-1) s(-1)) : 8.1 x 10(7) and 2.8 x 10(9) (X=4-CN), 7.5 x 10(6) and 3.7 x 10(8) (4-Cl), 7.8 x 10(6) and 8.4 x 10(7) (H), and 1.7 x 10(6) and 4.1 x 10(7) (4-Me). The reduction of the Co(III) complexes (NH3)(5)Co(X-pyO)(3+) by (.)CMe(2)OH is rapid (k=10(7)-10(8) L mol(-1) s(-1)) and acid-independent. On the basis of linear free energy relationships it is concluded that the reduction of the Co(III) complexes takes place by a chemical mechanism, whereby (.)CMe(2)OH reduces the coordinated X-pyO followed by an intramolecular electron transfer to the cobalt center.