The relative masking abilities toward the hydrolytic precipitation of manganous and cupric ions of some complexing agents were studied by means of pH titration method.Based on the experimental results, the sequences of the masking effect were obtained as following: for cupric ions, tartaric acid>citric acid>pyrocatecholdisulfonic acid> malic acid>sulfosalicylic acid>biuret, mandelic acid>sodium salicylate>trisodium ethylene diaminetetraacetate> semicarbazide hydrochloride>thiourea>sodium thiosulphate; for manganous ions, pyrocatecholdisulfonic acid>citric acid>tartaric acid> trisodium ethylenediamine tetraacetate>oxalic acid.It is advantageous to mask manganese in the divalent state, in the presence of some reducing agents (e.g.hydroxylamine, hydrazine), rather than in higher oxidation states, because manganese (Ⅱ) complexes are generally colorless.