A sedimentation study was made on the acid denaturation of bovine liver catalase in mediums of different salt concentrations. The native molecule dissociated into halves, partial ly between pH 3 and 4 and completely below pH 3. The sedimentation coefficient of the split product was greatly dependent upon the salt concentration and the pH value of the medium. The coefficient of the subunit first formed near pH 4 was 4.4–6.6 S and, on ac idifying the solution below pH 3, the coefficient decreased to a level of 1.9–4.4 S. The great drop of the coefficient on lowering pH or salt concentration was due to the unfolding of the structure of the half-size subunit and to the charge effect set forth by Pedersen. The process of dissociation was partially reversible, and a component having the same coefficient as that of the native molecule was recovered when the acidic solutions were dialysed against a buffer of pH 7.0. The results were discussed in connection with the subunit make-up of the catalase molecule, which was previously studied by the denauration with alkali and organic denaturation reagents.