Using SAXD and NMR techniques as well as visual observations, a detailed study of the extension of the phase regions and their structures has been performed for the ternary system 1-monooleoyl-rac-glycerol (MO)/n-OCtyl-beta-D-glucopyranoside (OG)/(H2O)-H-2 at 25 degreesC. OG in water forms a large region of a micellar solution phase, in which substantial amounts of MO can be dissolved. Increasing the MO concentration at constant water content results in the formation of two- and three-phase areas, consisting of liquid, lamellar, or cubic phases in equilibrium with a very dilute water/OG solution. Besides the different phases previously reported for the binary systems, an additional hexagonal phase occurs at high OG contents. Addition of minor amounts (approximate to1.5 wt%) of OG converts the cubic phases present in the MO/(H2O)-H-2 system to an Lalpha. phase, while the cubic phase in the OG/(H2O)-H-2 system is able to dissolve as much as 15 wt % MO. Since a major part of the phase diagram consists of planar bilayers, it is concluded that for most MO/OG ratios the spontaneous curvature is close to zero. The reason for this is discussed in terms of the molecular packing of MO and OG.