Abstract: The masking properties of malonic acid in chelometric titration have been investigated. Theoretically when EDTAis used as titrant, it can mask Fe(Ⅲ), Ti(Ⅳ) and Al, but if DCTAis used instead, it can mask Al only at pH 5-6.It was found experimentally that when DCTAis used as the titrant, malonic acid can mask Al up to 80mg or 60mg at pH 5.3 or 5.7. In order to indirect titration of Al, we had used malonic acid to displace the DCTAfrom the Al-DCTAcomplex at pH 5.5, but the displacement reaction was not complete even when the solution was heated or put aside for a long time. So we used the following procedure; Take two equal aliquots of Al solution Aand B. Only in solution A, add 1 to 2 g. of malonic acid to mask the Al. Then equal volumes of 0.02 M DCTA (over 2 ml in excess) are added to both solutions. After adjusting to pH 5.5 and using xylenol orange as indicator, titrate the remaining DCTAwith 0.02M Zn solution. The difference between the volumes of Zn solution in titrating the Aand Bsolutions multiplying the molarity will give the content of Al. In this way we found that: (1) in the determination of 5,8 or 10 mg Al, 1,1.5, or 2 g malonic acid should be used with accurate results; (2) in determining 5 mg of Al, 1 to 2 folds of the following metals do not interfere: Zr, Th, Sn, Bi, Fe, Ti, Ga, Ce, La, Y, Gd, Cr, Hg, Cu, Ni,Co, Pb, Zn, Cd, Mn, V, Sb, W, Mo, Ca, Mg, Ba, Sr, Ag. The selectivity of the titration is very high. It can be said that when DCTAis used as the titrant, malonic acid is a specific masking agent for Al.