This work is part of a long-term research effort to establish the relationship between lipid molecular structure and lyotropic and thermotropic mesophase propensity by comparing the equilibrium temperature-composition phase diagrams of a homologous series of monoacylglycerols, Here we report the temperature-composition phase diagram of monovaccenin (a C18:1c11 monoacylglycerol) in water, constructed using small-angle X-ray scattering in the temperature range of ca. 0-110 degrees C and the composition range of ca. 0-60% (w/w) water in the heating direction. The interpreted equilibrium phase diagram is based on several hundred discrete X-ray diffraction measurements in temperature-composition space recorded as a function of temperature in 5 degrees C increments and of composition in 4% (w/w) water increments on average. The phases identified and characterized structurally in this system include the lamellar crystal phase, the lamellar liquid crystal phase, two inverted cubic phases (Q(230), Ia3d; Q(224), Pn3m), the inverted hexagonal phase, and the fluid isotropic phase. The monovaccenin/water phase diagram is very similar to that of the monoolein (a C18:1c9 monoacylglycerol)/water system. However, there are important differences in transition temperatures and phase boundary positions between the two that we attribute to differences in the effective length and shape of the corresponding amphiphiles. The sensitive response to temperature and lipid identity of the average water channel radius in the fully hydrated, bicontinuous cubic phase makes monoacylglycerol systems important potential candidates for controlled drug release and membrane protein crystallization.