Three new heterocyclic azo compounds, 5(2-benzothiazolylazo)2,2 biphenyldiol (I), 5-(2-benzothiazolylazo)-8-hydroxyquinolene (II) and 4-(2-benzothiazo-lylazo)3-hydroxy-2-naphthoic acid (III) were synthesized. The formation constant of the reagents in 30% (v/v) ethanol and reactions with various metal ions were studied. These reagents (I-III) reacts with nickel ion to form (2:1) colored complex with an absorption band at 604, 635 and 643 nm. The apparent stability constants and the optimum conditions for complete color development were investigated. Beer's law is obeyed over the concentration ranges of 0.05-3.50, 0.05-4.00 and 0.05-3.10 micrograms ml-1. For more accurate analyses, Ringbom optimum concentration ranges were found to be 0.20-3.25, 0.20-3.80 and 0.20-2.90 micrograms ml-1 using reagents I, II and III, respectively. The apparent molar absorptivity and Sandell sensitivity were also calculated. The interference of various foreign ions on the determination of nickel was investigated. The proposed method has been successfully applied to the determination of nickel substrates in various biological samples.