A method for the preparation of metallic salts of fatty acids comprising the steps of: (a) reacting alkali hydroxide with a fatty acid in a saponification reaction to form an alkali soap; (b) reacting the alkali soap with an aqueous metallic salt solution in a double decomposition reaction to cause the production of metallic fatty acid salt; (c) performing at least one cycle of an alternating alkalinization-acidization reaction by adding an aqueous alkali hydroxide solution to the metallic fatty acid salt to raise the solution pH, followed by adding an acidic aqueous salt solution contain the same metal ions to lower the solution pH. High purity metallic salts of fatty acids can be obtained without a solvent washing step and the metallic fatty acid salts prepared from this invention can be effectively used as photolyzing agent to initiate and/or accelerate the photo-degradation of plastics.