Different intermolecular interactions in solutions produce various “mixing states”, such as ideal mixing, formation of clusters with the same component or another component and phase separation. The structure of a solution has been expressed by the radial distribution function from the microscopic standpoint. As a complementation for solution chemistry, a mesoscopic viewpoint is necessary to clarify the mixing state of a solution, because the mixing state is expressed in terms of fluctuations of the molecular distribution which are actualized in the mesoscopic special scale. The theoretical concept on fluctuations for solutions was first presented by Bhatia and Thornton. We established experimental methodologies to elucidate structures of solutions with fluctuations. For various solutions and mixtures in supercritical states, we obtained their density fluctuations, concentration fluctuations, particle-number fluctuations of individual components, and the Kirkwood-Buff parameters. As two examples, we present the mixing states of acetonitrile aqueous solutions and the ones of an n-pentane-water mixture in the supercritical state.