Abstract: A method of preparing an overbased lubricating oil additive comprising introducing carbon dioxide into a mixture containing (1) an oil-soluble organic acid or a metal salt thereof, which acid or salt includes a hydrocarbon group having 8 to 150 carbon atoms, and (2) an alkaline earth metal compound selected from the group consisting of oxides or hydroxides of alkaline earth metals, at a temperature of higher than about 100.degree. C, in a reaction medium of a lubricating oil, in the presence of a reaction accelerator comprising a dihydric alcohol, a phenol compound and elemental sulfur, thereby to effect complete carbonation; and thereafter removing all of the volatile components contained in the reaction product.