The microscopic structure of supercritical solutions of rare gases in water has been determined by neutron diffraction experiments using hydrogen isotope contrast on the water hydrogen atoms, at a concentration of 1 solute atom to similar to40 water molecules. It is found that even at this low concentration and high temperature, rare gases do affect the structure of water, giving a compression of the water-water spatial density functions, which is stronger for the less polarizable solutes. All the mixtures studied exhibit a water deficit around the solute. The sign of the derivative of the critical temperature of the mixture with respect to the critical pressure in the vicinity of the pure solute critical point seems to be correlated with the capability of the solute in penetrating inside the first shell of neighboring water molecules. (C) 2003 American Institute of Physics.