The solubilization of vitamin K-1 (VK1), a highly hydrophobic molecule, into a 1-monoolein/water (MO/W) system, is investigated by NMR self-diffusion, small angle X-ray scattering (SAXS), optical microscopy, and electrochemical methods. The various MO/W phases, namely L-2, L-alpha, C-G, and C-D, can accommodate different amounts of VK1. In particular, the L-alpha and the cubic C-G phases can solubilize up to 8 and 5 wt % VK1, respectively, without modifing the microstructure substantially. By contrast, the cubic C-D phase can accommodate only about 1 wt % VK1. Larger addition of VK1 produces a transition from the lamellar and cubic phases to a reverse hexagonal phase H-II, which in the MO/W binary system occurs only for the cubic phases and at temperatures above 80 degrees C. In practice the solubilization of VK1 induces almost the same phase transitions as would a temperature increase in the binary system. The SAXS and NMR self-diffusion data strongly suggest that the VK1 molecules are well intermingled with the PBO hydrophobic chains. Consequently the swelling of the C-G and L-alpha phases does not seem to be affected by the amount of VK1 present. However, if a sufficient number of VK1 molecules has penetrated into the lipid bilayer, the local change of the bilayer curvature is so large that a transition from cubic or L-alpha phase to a reverse hexagonal phase will occur. Electrochemical measurements indicate that, when solubilized in the cubic phase, the naphthoquinone group of VK1 reaches the bilayer/aqueous interface during the redox cycle, as the formal redox potential of the group is pH-dependent. The potential use of MO/W cubic phases with electrochemically active bilayer components in bioanalytical systems is discussed.