2-(2-Thiazolylazo)-4-methyl-5-(sulfoethylamino)benzoic acid (TAMSEB) and 2-(2-thioazolylazo)-4-methyl-5-(sulfopropylamino)-benzoic acid (TAMSPB) were synthesized. The acidity constants of these reagents and the formation constants of the copper(II) and nickel complexes were determined, and the effects of the length of sulfoalkyl groups on the properties as analytical reagent are discussed by comparing with those of 2-(2-thiazolylazo)-4-methyl-5-(sulfomethylamino) benzoic acid (TAMSMB). The sensitivity is high in the order of TAMSPB, TAMSEB and TAMSMB. The molar absorptivity of copper(II)-TAMSPB chelate is 58000 l mol−1cm−1. The spectrophotometric determination of copper (II) (25 ppb-25ppm) with a flow-injection system is described. Copper in Normal Control Serum was determi