Surface plasmon waves are perpendicularly polarised electro-magnetic waves which exist at boundaries between metals and dielectrics. The interest in surface plasmons from a measurement point of view arises from the fact that they can be excited optically and that their Oopagation is a sensitive function of the complex dielectric constants of the materials forming the interface. For chemical sensing, an interface is formed between the analyte (the dielectric) and a thin metal film. Surface plasmons are excited at this interface using a diverging laser beam which is reflected off the metal/analyte interface. Analysis of the reflected light shows a narrow dark line corresponding to energy being coupled to the surface wave. The position of this absorption line changes when the refractive index of the analyte changes. In our measurements, the analyte is a solution of acetone in water. By increasing the concentration of this solution its refractive index changes. By using an appropriate detector, the change in the position of the absorption line has been detected. Measurements made to deduce the concentration of the analyte by this technique are reported.