The equation of state of a charged phospholipid (dioleoylphosphatidylserine: DOPS, Na4+) in the absence of added salt is given at room temperature over six decades of osmotic pressure (100 to 2 x 10(8) Pa) covering the range of pressures where the different structures described previously have been observed.(1) It is compared to the universal expression given by the Langmuir equation and predictions for the Gouy-Chapman regime for highly charged bilayers. Below membrane separations of 10 Angstrom, the pressure of solid supported DOPS films is higher than the prediction of the Langmuir equation. In the linear swelling regime of flat bilayers and up to the regime where giant correlated fluctuations are present (the oyster shell state), the Langmuir equation holds better than the prediction given by Attard et al.(2) for the Gouy-Chapman regime. Close to maximum swelling (700 Angstrom), the pressure drops sharply. However, the universal power-law decay still holds in the unbinding regime, where the oyster shell state coexists with vesicles. The unbinding occurs as a progressive transformation of the oyster shell state into closed vesicles by a peeling mechanism of the lamellar domains.