The phase behavior and microstructure of mixed nonionic surfactant systems containing poly(oxyethylene) cholesteryl ether (ChEO(n), n = 15 and 10), a new alkanolamide-type foam booster, dodecanoyl N-methylethanolamide (NMEA-12), and water, were investigated at 25 degreesC by means of visual observation and small-angle X-ray scattering. In the ChEO(15)/water binary system, aqueous micellar (W-m), discontinuous cubic liquid crystal (I-1), hexagonal (H-1), rectangular ribbon (R-1), lamellar (L-alpha), and solid (S) phases are successively formed with increasing surfactant concentration. Although the R-1 phase is an intermediate phase formed in a very narrow composition range in conventional surfactant systems, its domain is unusually wider than that of H-1, which may be attributed to the packing constraint caused by the bulky cholesteric group in the lipophilic core of the aggregate. Upon addition of lipophilic NMEA-12 to the ChEO(15)/water binary system, the interfacial curvature of the aggregates decreases, and the micellar or liquid crystal phases formed in the binary system transform to the reverse micellar (O-m) phase via the L-alpha phase existing over a wide concentration range. The SAXS results establish an epitaxial relationship between the (11) plane of the R-1 phase and the (10) plane of the L-alpha phase. The ChEO(10)/NMEA-12/water system shows a phase diagram of similar general appearance, except that the W-m to R-1 phase transformation occurs via an optically anisotropic liquid crystal phase of unknown structure and the R-1 to L-alpha phase transition occurs through a narrow intermediate defected lamellar (L-alpha(H)) phase. The variation in the aggregate size and shape and the unit cell of the R-1 phase formed in ChEO(n)/NMEA-12/water systems is also discussed. (C) 2004 Published by Elsevier Inc.