Phase behavior, micellar structure, and cloud point have been investigated in water/long poly(oxyethylene)(n) oleyl ether (C18:1EOn) (n = 10. 7, 19.2, 30.1, and 50.8) systems. With increasing n, the discontinuous micellar cubic phase (I-1) becomes increasingly stable, effectively replacing hexagonal and lamellar liquid crystalline phases. From NMR self-diffusion measurements in the liquid micellar phase at lower concentrations, we found that micelles at larger n remain spherical and interact to a good approximation as hard spheres up to crystallization. In the cubic phase, small-angle X-ray data indicate a body-centered cubic arrangement. The aggregation number of the micelles monotonically decreases with increasing EO chain length, but the micellar molecular weight first decreases and then turns to increase at a certain EO chain length because of the increase of molecular weight of the surfactant molecule. As a result, the cloud point first increases with increasing EO chain length, reaches a maximum, and then slightly decreases as the EO chain length is further increased. Comparison was also made of the cloud-point behavior of poly(oxyethylene)(n) polymer systems with that of the C18:1EOn systems.