The adsorption behaviour of four members of the flavin family was examined in pH 7 solutions at Hg electrodes, both in the dark and when exposed to UV-visible light. In the absence of illumination, all flavins, which share the redox-active isoalloxazine ring system, adsorb strongly, initially in a parallel orientation and then undergo reversible, potential-driven orientational changes, from parallel to perpendicular, at higher surface coverages. Photolysis of flavin solutions (but not lumiflavin) yields two photoproducts, as detected electrochemically by their adsorption behaviour, and is supported by solution spectral analyses. The principal photoproduct is lumichrome, formed through an intramolecular photodealkylation reaction; it adsorbs strongly at Hg and forms a compact monolayer which undergoes a reversible redox reaction at ca. 100mV negative of the flavin potential. A second product, which oxidizes at ca. 150mV positive of the flavins and is detected electrochemically only when photolysis is carried out in the presence of oxygen, may result from an intramolecular photoaddition reaction involving the bridging of a hydroxyl group on the substituted chain to the isoalloxazine ring system.