A flavodoxin was isolated from the red macroalga Porphyra umbilicalis and the redox potentials for the oxidized-semiquinone ( E 2) and semiquinone-hydroquinone ( E 1) couples determined using a xanthine-xanthine oxidase reducing system in conjunction with Safranine O as a reference dye. The value of E 2 was − 273 mV at pH 7.0, while that for E 1, determined by calculating the semiquinone formation constant, was − 424 mV. Similar values were obtained for the flavodoxin from another red alga, Chondrus crispus. From these data, and knowing the association constant for the binding of FMN to the apoflavodoxin, the association constants for binding of the flavin semiquinone and hydroquinone could be calculated, and also the standard free energy changes for all the possible interactions. Since E 2 is pH dependent its redox potential at pH 8, the pH of the chloroplast stroma during light harvesting, is close to that of the NADP +-NADPH couple. Thus, at pH 8 the oxidized-semiquinone couple would be largely reduced, suggesting that this couple may be functional in NADP + photoreduction rather than the semiquinone-hydroquinone couple, generally assumed to be the physiologically important interconversion. The observations are of particular significance for Chondrus crispus since this organism lacks ferredoxin, which sustains this role in other organisms carrying out oxygenic photosynthesis.