BACKGROUND: MERCAPTANS ARE IMPORTANT AS ODORS, FLAVORS OR METABOLITES IN ENVIRONMENTAL, FOOD, PETROCHEMICAL AND BIOLOGICAL STUDIES [1, 2]. SEVERAL METHODS FOR THE DETERMINATION OF MERCAPTANS HAVE BEEN REPORTED, INCLUDING GC, HPLC, ATOMIC ABSORPTION, IDOMETRIC, ENTHALPIMETRIC, VOLTAMMETRIC AND TITRATION [3].SPECTROPHOTOMETRIC METHODS PROVIDE SUBSTANTIAL ADVANTAGES WITH RESPECT TO SIMPLICITY OF INSTRUMENTATION, AND SPEED OF ANALYSIS. WE DETERMINE OF SEVERAL MERCAPTANS USING THE UNIQUE CHARACTERISTIC OF SURFACE PLASMON GOLD NP. UNDER THE INFLUENCE OF ELECTROMAGNETIC RADIATION, THE ELECTRONS OF THE SURFACE ATOMS CAN EASILY MOVE THROUGH THE VACANT ORBITALS. THE COHERENT OSCILLATIONS OF THOSE ELECTRONS IN RESONANCE WITH THE FREQUENCY OF THE LIGHT GIVE RISE TO LOCALIZED SURFACE PLASMONS, WHICH CAN BE EXPLORED IN A WIDE RANGE OF APPLICATIONS IN CHEMISTRY, BIOLOGY AND NANOTECHNOLOGY. THE SURFACE PLASMON OSCILLATION OF GOLD NP IS DRASTICALLY CHANGED IF THE NANOPARTICLES AGGLOMERATE AT SOME EXTENT, ALLOWING THE INDIVIDUAL PARTICLES TO ELECTRONICALLY COUPLE TO EACH OTHER. AGGLOMERATION CAN GENERATE A NEW BAND ENCOMPASSING A RED SHIFT AND BROADENING, ASSOCIATED WITH THE LONGITUDINAL RESONANCE IN THE OPTICAL SPECTRUM. [4]. IN THIS PAPER, INTERACTION OF MERCAPTANS WITH GOLD NANOPARTICLES AND CHANGE OF UV-VIS SPECTRUM OF GOLD NP ARE USED TO DETERMINE OF TRACE AMOUNT OF THIS COMPOUNDS.METHODS: MERCAPTANS ADD TO GOLD NANOPARTICLES AND AFTER OPTIMUM OF CONCENTRATION OF GOLD NANOPARTICLES AND TIME OF INTERACTION OF GOLD NP WITH MERCAPTANS, UV-VIS SPECTRUM RECORDED AND LOW CONCENTRATION OF THEM (ABOUT PPB) IN MICROLITER SCALE (50 ML) WERE MEASURED.RESULTS: THE LOWEST MEASURED CONCENTRATION OF BUTANETHIOL, PENTANETHIOL AND PROPANETHIOL WAS ABOUT 16 PPB, 30 PPB AND 150 PPB, RESPECTIVELY.CONCLUSION: THE SENSIBILITY OF THE PLASMON RESONANCE TO THE ADSORBED SPECIES IS A KEY POINT FOR DEVELOPING APPLICATIONS OF GOLD NP IN CHEMICAL AND BIOLOGICAL SENSORS. THE TRACE AMOUNT OF MERCAPTANS WITH USING OF THE AGGLOMERATION OF GOLD NP WERE DETERMINE. THIS METHOD IS RAPID AND APPLICABLE TO SMALL SAMPLE.