X-ray diffraction data taken at high instrumental resolution were obtained for EPC and DMPC under various osmotic pressures, primarily at T = 30 degrees C. The headgroup thickness DHH was obtained from relative electron density profiles. By using volumetric results and by comparing to gel phase DPPC we obtain areas AEPCF = 69.4 +/- 1.1 A2 and ADMPCF = 59.7 +/- 0.2 A2. The analysis also gives estimates for the areal compressibility KA. The AF results lead to other structural results regarding membrane thickness and associated waters. Using the recently determined absolute electrons density profile of DPPC, the AF results also lead to absolute electron density profiles and absolute continuous transforms magnitude of F(q) for EPC and DMPC. Limited measurements of temperature dependence show directly that fluctuations increase with increasing temperature and that a small decrease in bending modulus KC accounts for the increased water spacing reported by Simon et al. (1995) Biophys. J. 69, 1473-1483.