A pressure-temperature phase diagram for 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) in a 50% (w/w) mixture with water has been constructed in the range 1-1200 bar and 65-90 degrees C. The lamellar gel (L(beta’)) phase prevails at high pressure and low temperature. It gives way successively to the lamellar liquid crystalline (L(alpha)) and inverted hexagonal (H-II) phases as pressure is lowered and temperature is raised. Measurements were made using synchrotron-based X-ray diffraction and a high-pressure beryllium cell. The pressure dependence of the L(beta’)/L(alpha) transition temperature, dT(m)/dP, is 25 +/- 2 degrees C/kbar. Compression/expansion in the lamellar phases was highly anisotropic. The isothermal pressure dependence of the chain-packing and lamellar repeat distance, (partial derivative d/partial derivative P)(T), in both the L(alpha) and L(beta’) phases was insensitive to temperature in the range studied. The average (partial derivative d/partial derivative P)(T) values for the lamellar repeat in the L(alpha) and L(beta’) phases and for the chain-packing repeat in the L(beta’) phase are 3.0 +/- 0.1, 1.18 +/- 0.06, and -0.117 +/- 0.04 Angstrom/kbar, respectively. In like manner, the isobaric temperature dependence of the chain-packing and lamellar repeat distance, (partial derivative d/partial derivative T)(T), in both the L(alpha) and L(beta’) phases is insensitive to pressure in the range studied. The average (partial derivative d/partial derivative T)(P) values for the lamellar repeat in the L(alpha) and L(beta’) phases and for the chain-packing repeat in the L(beta’) phase are -0.154 +/- 0.005, -0.053 +/- 0.003, and 0.0043 +/- 0.0002 Angstrom/degrees C, respectively. The absolute value of the ratio of the isothermal pressure dependence to the isobaric temperature dependence for all structural parameters measured, including area per chain, is remarkably close to the dT(m)/dP value reported above. A careful evaluation of mesophase structure parameters, a one-dimensional electron density profile, and a wide-angle diffraction profile has enabled us to describe in considerable detail the structure response of the L(beta’) phase in hydrated DHPE to temperature and pressure. Problems that can arise due to X-ray damage are discussed.