Diacylglycerol, a biological membrane second messenger, is a strong perturber of phospholipid planar bilayers. It converts multibilayers to the reverse hexagonal phase (HII), composed of highly curved monolayers. We have used x-ray diffraction and osmotic stress of the HII phase to measure structural dimensions, spontaneous curvature, and bending moduli of dioleoylphosphatidylethanolamine (DOPE) monolayers doped with increasing amounts of dioleoylglycerol (DOG). The diameter of the HII phase cylinders equilibrated in excess water decreases significantly with increasing DOG content. Remarkably, however, all structural dimensions at any specific water/lipid ratio that is less than full hydration are insensitive to DOG. By plotting structural parameters of the HII phase with changing water content in a newly defined coordinate system, we show that the elastic deformation of the lipid monolayers can be described as bending around a pivotal plane of constant area. This dividing surface includes 30% of the lipid volume independent of the DOG content (polar heads and a small fraction of hydrocarbon chains). As the mole fraction of DOG increases to 0.3, the radius of spontaneous curvature defined for the pivotal surface decreases from 29 A to 19 A, and the bending modulus increases from approximately 11 to 14 (+/-0.5) kT. We derive the conversion factors and estimate the spontaneous curvatures and bending moduli for the neutral surface which, unlike the pivotal plane parameters, are intrinsic properties that apply to other deformations and geometries. The spontaneous curvature of the neutral surface differs from that of the pivotal plane by less than 10%, but the difference in the bending moduli is up to 40%. Our estimate shows that the neutral surface bending modulus is approximately 9kT and practically does not depend on the DOG content.