Acetate retention was measured by treating suspensions of montmorillonite with C 14 ‐labeled acetate solutions, removing excess acetate by washing with methanol, and determining the acetate remaining by C 14 assay of the clay. The amount of acetate retained by montmorillonite was found to be dependent upon the concentration of salt used, pH of the system, and concentrations of competitive anions. Maximum acetate retention (0.05 meq acetate/g clay) occurred over the pH range of 2 to approximately 6. Increases in pH above 6 resulted in marked reductions in acetate retention. Under uniform conditions of pH (≅ 5.5), concentrations of approximately 2 n acetate salts were required to attain maximum retention, irrespective of salt source. Release studies showed OH ‐ , F ‐ , and H 2 PO 4‐ to be effective in removing sorbed acetate. On the other hand, SO 42‐ was only partially effective while NO 3‐ and Cl ‐ were ineffective in releasing acetate.