Abstract Laser flash photolysis has been used to compare the kinetics of reduction of ferredoxin isoforms from the green alga Monoraphidium braunii, and the ferredoxin and flavodoxin from the cyanobacterium Anabaena PCC 7119, by 5-deazariboflavin semiquinone (dRM) and the viologen analogue 1,l‘-propylene-2,2′-bipyridyI (PDQ +). Similar ionic strength-independent second-order rate constants (1.4 × 108M−1 s −1) were obtained for the reduction of both algal ferredoxin isoforms by dRM For the reduction of oxidized flavodoxin by dRM, a more complex behavior was observed, with a second-order rate constant for dRM. decay of 1.8 × 108M−1 S−1, and a first-order (i.e. protein concentration independent) rate constant of 450 s−1, that probably corresponds to the protonation of the FMN semiquinone cofactor, which occurs subsequent to electron transfer. A value of 5 × 107M−1 S−1 was obtained for the second-order rate constant of flavodoxin semiquinone reduction by dRM The reduction of ferredoxins and flavodoxin semiquinone by PDQ + showed nonlinear protein concentration dependencies, consistent with a minimal two-step mechanism involving complex formation followed by intracomplex electron transfer. A negative ionic strength effect on the kinetic constants was obtained, indicating the existence of attractive electrostatic interactions during electron transfer. With all the ferredoxins the k∞ values (rate constants extrapolated to infinite ionic strength) for the second-order step of the reduction process (complex formation) are smaller than previously reported for spinach ferredoxin, although Anabaena ferredoxin is somewhat more reactive than are the algal ferredoxins with the viologen. In contrast, the k∞ values for the first-order component of ferredoxin reduction (intracomplex electron transfer) for the algal ferredoxins are comparable to that for spinach ferredoxin, whereas for this reaction the ferredoxin from Anabaena has a smaller intrinsic reactivity. As compared with the ferredoxins, Anabaena flavodoxin has significantly smaller k∞ values for its interaction with the viologen analogue, both for complex formation and for electron transfer. In all cases the existence of nonproductive electrostatic interactions between the viologen analogue and the proteins is suggested by the data.