A surface acoustic wave sensor in which instrument drift resulting from accumulated surface contamination is minimized. The sensor includes a piezeoelectric substrate defined by an outer surface and a plurality of interdigitated electrodes mounted thereon. The electrodes are defined by one or more exposed portions and an unexposed portion abutting the outer surface of the piezoelectric substrate. An inert coating layer on the outer surface of the piezoelectric substrate and the exposed portions of the electrodes is provided, and can be a perfluoro-silane type compound, a perfluoro-trichloro-silane type compound, a perfluoro-acrylate type compound, polytetrafluoroethylene, or heptadecafluorodecyltrimethoxysilane.