The phase behavior of the diene lipid 1,2-bis(11,13-tetradecadienoyl)-sn-glycero-3-phosphorylethanolamine (DTDPE) and the corresponding phosphatidylcholine analogue (DTDPC) has been studied as a function of temperature (T) and relative humidity (RH) by means of infrared (IR) dichroism measurements. Lamellar/nonlamellar and solid/fluid phase transitions of lipid films spread on an ATR crystal are determined with high resolution by means of spectral parameters. The nonlamellar inverse hexagonal (H-II) phase has been found for DTDPE at T > 45 degrees C, a lamellar gel at T < 5 degrees C and the lamellar liquid-crystalline phase in between at RH > 90%. The ranges of the gel and the H-II states can be extended into the range of moderate temperatures by dehydrating the lipid where a direct transformation between the gel and H-II was found. The appearance of the H-II phase is somewhat unexpected in view of the short effective length of the tetradecadienoyl chains. It can be attributed to the influence of the terminal diene groups which remains however unspecified. Similar to other PE lipids, DTDPE tends to transform from the gel into crystalline polymorphs. The PC analogue assembles into liquid-crystalline membranes at nearly all conditions studied. The hydration, conformation, and packing of the polar region of the lipid aggregates has been characterized by IR spectral parameters. This method has been complemented by P-31 NMR to assign the nonlamellar phase and by DSC to yield thermodynamic information.