a method for the preparation of metallic salts of fatty acids comprising the steps of : reacting alkali hydroxide with a fatty acid in a saponification reaction to form an alkali soap ; reacting the alkali soap with an aqueous metallic salt solution in a double decomposition reaction to cause the production of metallic fatty acid salt ; 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 .