Some thiazolylazo- and pyridylazobenzoic acids were studied for the metallochromic indicator in EDTA titration of nickel:2-(2-thiazolylazo)-4-methyl-5-(sulfomethylamino)benzoic acid(TAMSMB), 2-(5-bromo-2-thiazolylazo)-4-methyl-5-(sulfomethylamino)benzoic acid (5-Br-TAMSMB), 2-(2-thiazolylazo)-5-(N-ethyl-N-sulfomethylamino)benzoic acid(N-Et-TASMB) and 2-(3, 5-dichloro-2-pyridylazo)-4-methyl-5-(sulfopropylamino)benzoic acid(3, 5-diCl-PAMSPB). The formation constants of the nickel chelates with these reagents and the rate of the substitution reaction of nickel-TAMSMB chelate with EDTA were determined. TAMSMB is an excellent indicator for nickel because of the good color contrast between indicator and the nickel chelate, the large rate of color change reaction at equivalence point and the easy preparation of the reagent.