Dilauroyl and dimyristoylphosphatidylglycerol (DMPG) form a more stable gel state when aqueous suspensions are incubated several days at low temperature (0-2 degrees C), pH 7.4 with 0.15 M NaCl. This gel state is characterized by a higher transition temperature and a higher transition enthalpy. The geometry of this gel state is distinguishable from the metastable gel state that forms rapidly upon hydration on the basis of its x-ray diffraction pattern. Infrared spectra in the CH2 scissoring region indicate that the stable gel phase of DMPG is also characterized by reduced reorientational fluctuations of acyl chains and increased interchain interactions. Analysis of vibrational bands due to ester carbonyl groups of DMPG suggests that the transition to a new gel phase is initiated by changes in the interfacial and/or headgroup region of the bilayer, most likely via formation of interlipid hydrogen bonds. The melting of the stable gel phase of DMPG is accompanied by a gross morphological change resulting in vesiculation.