The conformation of the alkyl chain in alpha-dodecyl-omega-hydroxytris(oxyethylene)(C(12)E(3)) in water has been studied by C-D stretching infrared spectroscopy. This conformational analysis is based on the fact that the wavenumbers of the isolated C-D stretching vibrations are sensitive to the conformation in the vicinity of the C-D bond. Infrared spectra were measured for five selectively monodeuterated species of C(12)E(3), namely CH3(CH2)(11-k)CHD(CH2)(k-1)(OCH2CH2)(3)OH, where k = 1, 2, 4, 6, and 8, in the lamellar (L(alpha)) and isotropic solution (L(2)) phases. The C-D stretching bands for each of the monodeuterated species were assigned to particular conformations of the specifically deuterated part of the chain. From the observed intensities of the C-D stretching bands, the fractions of the trans conformation around the dodecyl C-C bonds and the oxyethylene-adjoining O-C bond and the fractions of the consecutive trans conformations around the two adjoining bonds were evaluated. The conformational change at the phase transition from L(2) to L(alpha) is not significant and only a small increase in the trans fraction is observed for the C-C bonds close to the alkyl/oxyethylene interface. This implies that the conformational states in the L(2) and L(alpha) phases in the vicinity of their boundary are substantially not different. In the L(alpha) phase, when the composition or the temperature approaches the region of the phase separation or transition, the trans fractions for the C-C bonds closer to the alkyl/oxyethylene interface and those closer to the chain terminal decrease significantly. These observations indicate that the conformational transformation from trans to gauche at these chain positions makes the lamellar structure less stable and leads eventually to the structural destruction. The fraction of the consecutive trans conformations may be used as a measure of the conformational order at particular positions of the chain. The present results show that the ordering is the highest in the middle of the chain in the L(alpha) and L(2) phases. This vibrational spectroscopic observation, together with the previous NMR observations, indicates that the alkyl/oxyethylene interface is flexible with respect to the conformation and the orientation of the chain.