A method for the determination of mercury in environmental and biological samples has been developed. The method is based on the complexation reaction between mercury and 4-(-2-benzothiazolylazo) 2,2biphenol (I)5-(2-2-benzothiazolylazo)-8-hydroxyquinoline (II), 1-(2-benzothiazolylazo)2,3-naphthalinediol(III), and 8-(2-benzothiazolylazo) 1,4-naphthalenediol (IV). The potentiality of the prepared compounds as new chromogenic reagents for the spectrophotometric determination of mercurt ions are studied by extensive invesigatios of the optimum conditions favouring the formation of the colored complexes. The effect of various interfing ions and their tolerance limits has been studied. The conditions developed were used successfully for the determination of Hg2+ ions in environmental and human (urine and blood) samples. Annali di Chim., 90, 389–399 (2000).