The stability of negative Sb2S3 sol of mean particle diameter 87.5 A has been ascertained in the pre­sence of K+, Ba2+, Al3+ and Th4+.The electrophoretic measurements of the sol permitted the calculation of its zeta potential (assumed to be equal to the electrostatic potential) as a function of ionic strength. From the knowledge of potential the energy of repulsion, VR, was calculated; the attraction energy, VA, was estimated from physico-chemical data. Combination of the two energies yielded the profiles of interaction from which stability ratio could be calculated. Stability factor, F, was also estimated from the Derjaguin equation, but it was not satisfactory presumbaly owing to the exclusion of VA. The flocculation powers of cations were in the order A13+> Ba2+> Mn2+> Th4+> k+> Na+. Thorium behaves anomalously perhaps due to hydrolysis. In general, the interaction between the Sb2S3 particles could be interpreted in the light of the D.V.O. theory.