From a review of the major factors responsible for surfactant mesophase structure, a model phase diagram is deduced which shows phase structure as a function of surfactant volume fraction and micelle curvature. To test this model the phase behaviour of a series of pure polyoxyethylene surfactants (CnEOm) with water has been studied using optical microscopy over the temperature range 0–100 °C. The compounds studied were C8EO3, C8EO4, C8EO8, C8EO12, C10EO3, C12EO3-C12EO6, C12EO8, C12EO12, C14EO3, C14EO6, C16EO3, C16EO4, C16EO6, C16EO8, C16EO12, C9PhEO8 and C12(2-C10)EO10. Phase diagrams were determined for C8EO4, C12EO3-C12EO6, C12EO8, C16EO4, C16EO8 and C16EO12. With the other compounds optical microscopy was used to determine the number, sequence and type of mesophases. The mesophases observed were cubic–spherical-micelles (I1), hexagonal (H1), normal-cubic–bicontinuous (V1), lamellar (Lα) and reversed-cubic–bicontinuous (V2). Large head groups and low temperatures favour I1 and H1 phases, while Lα and reversed phases occur for small head groups and higher temperatures. There is agreement between experiment and theory for low to medium temperatures if increasing temperature is assumed to lead to a decreased surface area per molecule at the micelle surface (a). At high temperatures and low water content theoretical concepts were reconciled to practical behaviour only by assuming that increased interactions between EO groups occur at a critical water concentration. Two separate mechanisms are proposed for the lower consolute behaviour of surfactant + water solutions (the cloud point). One, involving van der Waals attractions between micelle cores, operates at low temperatures, while the second, involving intermicellar EO—EO attractions occurs at high temperatures. These two mechanisms can account for the ‘double’ cloud point phenomenon observed for surfactants with short EO groups.