The lyotropic phase behavior of the dienic lipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) has been investigated by means of IR spectroscopy at 25 degrees C. Gradual hydration has been realized exposing the lipid to an atmosphere of variable relative humidity (RH). Upon scans of decreasing RH, the liquid crystalline lipid undergoes the chain-freezing transition to a metastable gel state. By storage of the sample at low RH, the gel transforms to a crystalline subgel designated as SG(I). Subsequent hydration induces the conversion to a second subgel (SG(II)). The subgel phases are characterized by the dense packing of the acyl chains as indicated by the correlation field splitting of the CH2 rocking and bending modes. Band shifts of phosphate group vibrations as well as the splitting of the carbonyl stretching mode are correlated with the hydration of the polar region of the bilayer given in terms of the molar ratio of water to lipid. The nu(1,3)(OH) absorption band of water yields qualitative information about the water-lipid interaction. The drastic sharpening of this band in the SG(I) phase was attributed to the reduction of water binding sites on the lipid, leading to a more uniform population of water molecules adsorbed onto the lipid headgroup. The external conditions of phase transformation of DODPC were compared with corresponding data of dimyristoylphosphatidylcholine (DMPC) having the same number of subsequent methylene segments in the acyl chains. Apparent differences can be attributed to the influence of the diene groups representing a rigid spacer inserted between the methylene chains and the ester groups of the lipid, i.e., in a position near the polar/apolar interface of the bilayer.