Optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and nuclear magnetic resonance spectroscopy have been used to study the lyotropic phases formed by trioxyethylene glycol monohexadecyl ether with water (2H2O). The phase diagram exhibits regions corresponding to lamellar (Lα), inverse cubic [V2(1), V2(2)], gel (Lβ) and isotropic liquid phases (W, L2, L3). The two V2 phases were identified by both optical microscopy and small-angle X-ray measurements. The gel phase exhibits a large melting entropy, and part of the molecule has a much restricted mobility. Low-angle X-ray data indicate that the alkyl chains are ordered, with a thickness equivalent to one hydrocarbon chain length. Taken together these data confirm a monolayer ‘interdigitated’ alkyl chain structure for the gel. In the presence of water the gel is stable (at thermodynamic equilibrium) below 40 °C. Anhydrous C16EO3 exhibits polymorphic phase behaviour. After melting the crystalline surfactant, a metastable gel structure forms on cooling and the crystalline solid reforms only after lengthy storage at low temperature. The maximum water-layer thickness of the Lα phase is larger than that of Lβ despite the expected weaker inter-layer attractions in the latter case. This implies the existence of an additional repulsion in Lα, possibly arising from elastic undulations of liquid bilayers.