Conformational and orientational characteristics of amphiphiles incorporated in the hexagonal, lamellar, and reversed hexagonal aggregates formed by a ternary lyotropic system of sodium octanoate, 1-decanol, and water have been investigated. Deuterium NMR quadrupolar splittings observed from the randomly and selectively deuterated compounds were simulated by a combined use of the rotational isomeric state scheme and the maximum entropy method. For the individual mesophases, the bond conformations and orientational order parameters of octanoate and 1-decanol were evaluated and related to the aggregate structures. In the lamellar phase, in particular, the 1-decanol chain was found to exhibit peculiar conformations; the C-C bond nearest the OH group is fixed in the gauche state, the adjacent C-C bond also shows a gauche preference, and the second C-C bond from the methyl terminal has a high gauche probability. It follows that the chain length of 1-decanol is almost equal to that of octanoate in the all-trans conformation. Ab initio molecular orbital calculations at the MP2/6-31+G*//HF/6-31G* level for 1-butanol have revealed the inherent conformational preference of the head portion of the alcohol chains; the first C-C bond from the headgroup prefers the gauche state to the trans state by 0.27 kcal mol(-1), and the g(+/-) g(+/-) states in the first and second C-C bonds from the OH group have a comparatively small free energy of 0.43 kcal mol(-1) relative to the tt state.