Aqueous solutions of the nonionic surfactant C(8)E(4) With different mass fractions of the amphiphile have been studied by small-angle X-ray scattering (SAXS) with dependence on temperature. The demixing temperatures of the samples were determined by viscosity and ultrasonic speed measurements. The temperature dependence of measures shows discontinuities at demixing of the sample. Below the demixing temperature the viscosity measurements indicate increasing size of the solute particles. The SAXS curves can be well described throughout the whole temperature and composition range with the model of a inhomogeneous sphere with a core radius of about 1 nm. This value is in good agreement with the theoretical length of an all-trans-octyl chain. The combination of these results with the viscosity data supports the model of aggregating spherical micelles.