The electrochemical behaviour of vitamin K-1 (VK1) incorporated in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) deposited on a hanging mercury drop electrode was investigated by a computerized chronocoulometric technique. The kinetics of VK1 electroreduction to the corresponding quinol, VK1H(2), and that of VK1H(2) reoxidation to VK1 were examined for reactant concentrations ranging from 0.5 to 2 mol% by varying the pH from 5.5 to 9 with phosphate and berate buffers. On the basis of a general kinetic approach it was concluded that the reduction of VK1 to VK1H(2) in a DOPC monolayer takes place via the reversible uptake of one electron, yielding the semiquinone radical anion VK1(.-), followed by the rate determining protonation of the latter. On the other hand, the oxidation of VK1H(2) to VK1 takes place via the reversible release of one electron, yielding the semiquinone radical cation VK1H(2)(+.), followed by the rate determining deprotonation of the latter. The only effective proton donors in VK1 reduction are the protons, whereas the main proton accepters in VK1H(2) oxidation are the water molecules.