The volume changes at 25° C. on mixing two solutions of equal ionic strength were determined for the six combinations of the four electrolytes, LiCI, NaCI, Li2SO4, and Na2SO4 at ionic strengths of 1 and 4. Single-mixing experiments were made at an ionic strength of 2.25 for the six combinations, and at ionic strengths of 0.25, 0.50, 1.5, and 3.0 for the Na2SO4-NaCI system in order to determine the concentration dependence of the volume changes. The volume changes on mixing the heterionic solutions (Na2SO4-LiCI and Li2SO4-NaCI) could be calculated from the volume changes observed in homoionic solutions to within 0.03 ml. per equivalent. © 1968, American Chemical Society. All rights reserved.