The kinetics of electron transfer and oxygen evolution at citrate-stabilized IrO(x)(.)nH(2)O colloids were studied by time-resolved UV-visible spectrosopy and by steady-state photolysis of [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridyl) and persulfate in a hexafluorosilicate/bicarbonate buffer. Time-resolved studies of the reaction of [Ru(bpy)(3)](3+) with these colloids show an initial fast electron transfer, corresponding to oxidation of Ir(III) to Ir(IV). Further oxidation of surface Ir atoms occurs concomitantly with oxygen evolution with a second-order rate constant of 1.3 x 10(6) M-1 s(-1). Both the time-resolved reduction of [Ru(bpy)(3)](3+) by IrO(x)(.)nH(2)O and the photocatalytic oxygen evolution under non-light-limited photolysis conditions have a H/D kinetic isotope effect (KIE) of 1.0. This contrasts with significantly higher KIE values for oxygen evolution from molecular cis,cis-[(bpy)(2)Ru(OH2)](2)O](4+) and [(terpy)(H2O)Mn-III(O)(2)(OH2)terpy)](3+) water oxidation catalysts. This is consistent with the conclusion that, under the conditions of most photocatalytic experiments (similar to10(-4) M [Ru(bpy)(3)](2+) concentration), electron transfer from the colloid to the oxidized sensitizer rather than formation of a surface-bound hydroperoxy species is the rate-determining step in photocatalytic oxygen evolution.