Abstract—Laser flash photolysis spectroscopy has been used to investigate the kinetics of oxidation and reduction of pho‐tosynthetic plastocyanin (Pc) and cytochromec6(Cyt) from the cyanobacteriaAnabaenaPCC 7119 andSyne‐chocystisPCC 6803 by lumiflavin, riboflavin and flavin mononucleotide (FMN). For both redox reactions similar steric and electrostatic effects were observed withSyne‐chocystisproteins andAnabaenaCyt, thus suggesting that the same (or closely adjacent) sites are being used for electron entry and removal. In the case ofAnabaenaPc, however, both the steric and electrostatic effects suggest that FMN‐dependent protein oxidation and reduction may occur at different sites, that is oxidation at the hydrophobic patch and reduction at the hydrophilic one. KineticpKavalues of 5.8 and 6.2 have been determined for the lumiflavin‐dependent reduction ofAnabaenaandMonoraphidiumPc, respectively, whereas the oxidation reactions appear to be pH independent. Not only the reduction kinetics but also protein tyrosine fluorescence quenching by iodide ions suggest the occurrence of pH‐induced conformational changes in Pc. In conclusion, kinetic and fluorescent studies indicate that there are considerable quantitative similarities between Cyt and Pc when isolated from the same organism, consistent with the identical physiological role that these two proteins play ins