The phase diagram of a nonionic surfactact, namely Synperonic A7 (C-13/C-15 alcohol with 7 mol of ethylene oxide) was investigated using polarizing microscopy, differential scanning calorimetry(DSC), and theological measurements. Using polarizing microscopy, the various phases produced were investigated from the texture observed at various surfactant concentrations. At 20 degrees C, the surfactant produced hexagonal (middle phase) structures when the concentration was increased above 30%. This middle phase structure changed to a lamellar (neat phase) structure at concentrations above similar to 55%. The neat phase remained up to a concentration of similar to 85%. Microscopic investigations were also carried out as a function of temperature (at constant surfactant concentration). This established the temperature at which phase transition from liquid crystalline structure to isotropic liquid phases occurred. Some results on these phase changes were also obtained using DSC measurements. Both steady state and oscillatory measurements were carried out as a function of surfactant concentration and temperature. At surfactant concentrations below 30%, i.e. in the L(1) region, the viscosity-temperature curves showed a maximum that shifted to lower temperatures as the surfactant concentration increased. The initial increase in viscosity with increase of temperature was attributed to aggregation and/or change in the shape of the micelle, whereas the reduction was thought to be due to the breakdown of the micellar structure. Viscoelastic measurements of the concentrated surfactant solutions (30-85%), i.e. in the hexagonal and lamellar regions, were carried out at 20 degrees C. The moduli-concentration curves showed two maxima at similar to 45 and similar to 70% and this was explained in terms of the structural changes that occur in the system as the concentration was increased. Measurements as a function of temperature showed a rapid reduction in the moduli values at a critical temperature at which the liquid crystalling structure undergoes "melting" to form a liquid-like structure. The theological results were used to establish the phase diagram and the results were compared with those obtained using microscopy and DSC.