The structures of liquid Hg-Rb alloys were studied as a function of composition with neutron diffraction. In the intermediate Rb concentration range, the obtained structure factors show a small prepeak, which may be evidence of the formation of Hg polyanion units in liquids. The reverse Monte Carlo (RMC) analysis was applied to separate the total radial distribution function into corresponding partial radial distribution functions. Up to 10 at.% Rb, no obvious changes are found for the first peak position of the partial radial distribution functions of the Hg-Hg pair and that of the Hg-Rb pair. Above 20 at. % Rb, the increase of the first peak position between the Hg-Rb pairs appears. At 60 at. % Rb, the nearest neighbor distance between Hg and Hg shows the closest value in the concentration range studied. These results indicate that with the proceeding of charge transfer the solvation structure in the dilute Rb concentration range changes into the structure containing the polyanion formation, composed of Hg species.