The solubilities of o-nitrobenzoic acid in aqueous solutions of NaCl, KCl, LiBr, NaBr, KBr, KNO₃, KSCN, NaClO₄ and (CH₃)aNBr were determined at 25℃.Following Randall and Failey, some assumptions about the activity coefficients of the ions of o-nitrobenzoic acid in salt solutions were made and thereby it was possible to calculate the activity coefficients of its undissociated molecules in these solutions.In the range of salt concentrations C, studied (the least being 0 to 0.5 n, the largest being 0 to 1.5 n), the plots of log f_u vs.C_s are all straight lines.Comparison of their slopes yields the salting-out order of the undissociated part of the o-nitrobenzoic acid as follows:NaCl>LiBr>NaBr>KCl>KBr>KNO₃>NaClO₄>KSCN>(CH₃)₄NBr,in which the order between potassium nitrate and sodium perchlorate is correct, only when our assumptions about the activity coefficients are correct.