Rubidium salts (RbCl, RbNO3, Rb2SO4, and Rb2CO3) + water + ethanol systems have been comparatively investigated and discussed. The solubilities and mutual miscibilities decrease with increasing ethanol content in the systems. The equilibrium solid phases for the RbCl-, RbNO3-, and Rb2SO4-containing systems are simple nonhydrates, but that of the Rb2CO3-containing system is Rb(2)CO(3)(.)2H(2)O. Rb2CO3 can make the miscible aqueous ethanol mixture split, and the three others cannot. An empirical formula was proposed to correlate the experimental data, and the phase diagrams were constructed. An explanation for the experimental results based on molecular structure is postulated.