Two effective methods to obtain density fluctuation, which is one of suitable parameters describing the inhomogeneous structure of supercritical fluids, are small-angle X-ray scattering (SAXS) experiments and thermodynamic calculations using an equation of state. The strengths and weaknesses of each method are discussed by comparing the density fluctuations of supercritical Co-2, CHF3 and H2O. Although their density fluctuations obtained from SAXS measurements were reported in previous papers, we have performed the experiments in better conditions, and the results are listed as the most reliable data. (C) 2003 Elsevier B.V. All rights reserved.