A theoretical model is developed to describe the temperature and pressure-induced phase transitions of phospholipid bilayer membranes simultaneously. By means of this model the phase transition enthalpies,-temperatures,-pressures and volume changes of different phospholipid membranes are calculated and compared with the available experimental data. The p-T diagrams of the membranes are obtained, too. Phase diagrams for binary mixtures with components differing in chain length by 2 and 4 carbons are determined at different pressures. It is found that diagrams shift without significant deformation in their shape towards higher temperatures with increasing pressure.