ANEW reagent, (2E,2'Z) -2,2'- (3- ((E)- (2-hydroxyphenyl) ..........diazenyl) pentane-2,4- diylidene) bis (hydrazinecarbothio-amide) (OPTS) was synthesized and studied. Acid-base, spectrophotometric properties of OPTS were studied in 50% ethanol-water mixture solutions at pH 3–11. Dissociation constants of the reagent OPTS have been determined by spectrophotometry and potentiometric titrations: pK1=9.60±0.02; pK2=10.45±0.05 in its computational and graphical versions at an ionic strength 0.1M. The stability constants of the Hg-OPTS are logK1=6.18±0.04 and logK2=5.22±0.05. The reaction of Hg(II) with OPTS gives mononuclear complex in 50% ethanol - water mixture solution with λmax = 448 nm at pH 8.5. The effects of foreign ions and masking agents on the determination of Hg(II) with the new reagent were monitored. The mono complex obey the Beer law in the Hg(II) concentration range 5.10–50.17 mg/25ml. Molar absorption coefficients was determined. In this paper, we report the synthesis of the novel Hg(II) complexes with OPTS. The complexes were characterized by UV-vis and IR spectroscopy, elemental analysis, molar conductivity and thermal decomposition.