Open AccessEven though the interaction of a < 1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (kBT), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub‐millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo‐excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green’s function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin‐selective recombination reactions, and the effects of the inter‐radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field‐dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ~500 T magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin‐containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes.