The phase behavior, microstructure and rheological properties of mixtures of polyoxyethylene type nonionic surfactants (C12EOn, n = 0–4) and alkyltrimethylammonium bromide cationic surfactants in water were investigated. Upon addition of C12EO3 to the aqueous cationic system at high surfactant concentration, both the interfacial curvature of aggregates and the effective cross-sectional area per surfactant molecule, aS, decrease, resulting in a hexagonal-lamellar liquid crystal phase transition. This effect becomes more pronounced as the alkyl chain-length of the cationic surfactant increases. At low surfactant concentration, addition of nonionic amphiphile to micellar solution above a particular concentration, C*, induces a rapid unidimensional micellar growth, and particularly in CTAB systems viscoelastic micellar solutions of entangled wormlike micelles are formed. With successive addition of the nonionic amphiphile ultimately a micellar to lamellar phase transformation occurs. Rheological measurements within the Wm-phase show that with decreasing EO chain-length, C* also decreases and viscosity increases more swiftly with increasing concentration of nonionic amphiphile, suggesting a rapid micellar growth. This is attributed to the decrease in aS, as predicted by a simple geometrical model.