Small-angle X-ray scattering measurements have been carried out on tert-butyl alcohol (TBA)-water mixtures at 28, 40, and 55°C. The temperature effects in the concentration fluctuation and the Kirkwood-Buff parameters have been studied. Debye's correlation length has also been determined for each temperature. With the increase of temperature, the fluctuation becomes larger and the correlation length becomes longer, which indicates that the microscopic demixing of the mixture proceeds as the temperature is raised. The temperature and concentration dependences of the mixing state of the TBA-water solution are discussed.