Photoexcited [Ru(bpy)3](2+) (bpy = 2,2’-bipyridine) can be quenched with [Co(NH3)(5)Cl](2+) to give [RU(bpy)(3)](3+); this photogenerated oxidant (E(0) = 1.25 V vs NHE) reacts with horseradish peroxidase isoenzyme c (HRPc) to produce oxidized protein species. Spectra and kinetics measured by laser-flash transient spectroscopy show that oxidation of the trivalent resting state, [PFeIII](+) (P = porphyrin dianion), to ferryl compound II, [PFeIV = O], is preceded by generation of a;R-cation porphyrin radical intermediate, [PFeIII](. 2+). In the interval 7.8 < pH < 9.8, the rate-limiting step for the transformation of the radical intermediate to compound II is the binding of a water molecule to the five-coordinate heme iron, k(obsd) = (4.1 +/- 0.9) s(-1); this step is followed by fast proton and electron transfer to give the ferryl species. There is a burst in compound II formation in the pH region (10.3 < pH < 10.8) in which the heme iron changes from a five-coordinate, high-spin species to a six-coordinate, low-spin complex (pK(a) = 10.9); this burst is attributed to very rapid conversion of a hydroxo-ligated ferric pi-cation radical porphyrin to a ferryl species. The rate constant for the porphyrin-centered oxidation of compound II to compound I ([PFeIV = O] to [PFeIV = O](.+); k = 1.1 x 10(8) M(-1) s(-1)) is slightly larger than that for the oxidation of [PFeIII](+) to [PFeIII](. 2+) (k = 2.5 x 10(7) M(-1) s(-1)) at pH 10.3; both porphyrin-centered oxidations are much faster than the conversion of [PFeIII](+) to [PFeIV = O] below pH 9.8.