Lamellar and hexagonal (H-I) liquid-crystalline mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) with the nonionic detergent octaethyleneglycol mono-n-dodecyl ether (C(12)E(8)) have been studied by solid-state H-2 and P-31 NMR spectral techniques. The HI phase structure is considered as a useful model for small mixed detergent/phospholipid micelles. Formation of such mixed micelles is an obligatory step in the investigation of biological membranes and membrane proteins. Both the phospholipid P-31 NMR chemical shift anisotropy and the H-2 NMR quadrupole splittings can be observed in the hexagonal mesophase, in contrast to micelles where motional averaging precludes the direct observation of residual chemical shift or quadrupolar tensors. The H-2 NMR spectra were evaluated in terms of carbon-deuterium bond order parameter profiles. The order profiles obtained in the lamellar detergent/phospholipid mixtures were significantly different from the corresponding profiles in the mixed detergent/phospholipid H-I phase. In the lamellar phase; an increasing proportion of C(12)E(8) led to a significant reduction in the absolute magnitude of the individual order parameters, whereas in the H-I phase the order parameters remained relatively constant Over abroad range of detergent/phospholipid molar ratios. It was also shown by P-31 and H-2 NMR that the interfacial segments of the phospholipid (the glycerol backbone and the headgroup dipole) assume an almost identical conformation in the lamellar and in the H-I phase. Average chain lengths and mean cross-sectional areas were derived from the order parameter profiles and interpreted in terms of geometrical properties of the hexagonal and lamellar phase aggregates. Furthermore, measurements at different temperatures yielded an estimate of thermal expansion coefficients of the phospholipid acyl chains in the different phases. It is concluded that addition of detergent to lamellar phospholipids results in increasing packing constraints which are eventually relieved by a lamellar --> H-I transition. These packing constraints are discussed in terms of the molecular shape concept and with reference to statistical mechanical models of chain packing in bilayers, H-I-structures, and small micelles.