The relative masking abilities toward the hydrolytic precipitation of bismuth and zirconyl ions of some complexing agents were studied by means of pH titration method.Based on the experimental results,the sequences of the masking effect are obtained as following: for bismuth ions,pyrocatecholdisulphonic acid>tartaric acid,cystein > citric acid > ethylenediaminetetraacetic acid > dimercaptopropanol > sulphosalicylic acid > sodium thiosulphate > potassium iodide > thiourea > potassium thiocyanate; for zirconylions,pyrocatecholdisulphonic acid > citric acid > tartaric acid > ethylenediaminetetraacetic acid > malic acid > sulphosalicylic acid > sodium carbonate > sodium fluoride,pyrocatechol,inositol>sodium salicylate,cystein> oxalic acid.