Li2SO4-Li3O4 system was studied using classical methods : DTA, X-ray powder diffraction at several temperatures and electrical conductivity measurements. As found in Li2SO4-Li3PO4 system, there is no intermediate compound but only primary solid solutions. The boundaries of these phases are very different in those two systems and lead to very different phase diagrams. The solid solubility of Li3VO4 in high temperature form of Li2SO4 (alpha) is weak, while the solid solubility of alpha-Li2SO4 in high temperature forms of Li3VO4 (gamma and delta) is stranger. The limit of the gamma-Li3VO4-type solid solution namely Li3-xV1-xSxO4 is obtained at 835 degrees C with x = 0.3 (eutectic). At low temperature (T less than or equal to 500 degrees C) the ionic conductivity in the two-phase region grows when Li3VO4 is added up to 40 mol% while at high temperature (T greater than or equal to 600 degrees C) a small decrease is observed. This last result agrees with the "paddle-wheel" mechanism of ionic mobility.