Temperature and pressure dependences of the structural fluctuations have been studied for methanol-water mixtures in the whole range of mole fractions by small-angle X-ray scattering (SAXS). A sample cell, designed for stable operation at high temperature and pressure retaining a constant path length, has enabled a precise measurement of absolute SAXS intensities, from which fluctuations in concentration and density have been observed. The results are compared with those measured at ambient conditions. The fluctuations in the density increase significantly with the rise in temperature and pressure, whereas those in the concentration remain essentially unchanged. (C) 2004 Elsevier B.V. All rights reserved.