The influence of charge density and hydrocarbon chain length on the sponge-vesicle transformation of two bilayer-forming ionic phospholipids, the sodium salts of dilauroyl- and dimyristoylphosphatidylglycerol (NaDLPG and NaDMPG), was examined by measuring the solution properties of the lipids in water. The phase diagrams of these compounds in water indicate they undergo a transformation from a transparent, jelly-like sponge phase to unilamellar vesicles at a critical temperature T*; for NaDLPG T* = 19 degrees C, and for NaDMPG T* = 31.6 degrees C. At T > T* multilamellar vesicles form. The Krafft temperature for NaDLPG is similar to 5 degrees C, just above, T-m, the gel-liquid crystal transition temperature, and for NaDMPG it is similar to 32 degrees C, slightly higher than T*. For both lipids, the charge density of the equilibrium monolayers at the air/water interface decreases dramatically as T increases and approaches T*. This effect is attributed to hydrolysis of the phosphate moiety of the lipid. Changes in solubility consistent with a decrease in bilayer charge density at T* are also observed. The transformation from the sponge phase to vesicles at T* is a structural response to the change in bilayer charge density. The results emphasize the importance of bilayer-localized chemical reactions in the transformation of the sponge state to vesicles.