Factors influencing the elastic properties of polymeric coatings on surface acoustic wave (SAW) substrates were investigated. The study was initiated because of the need to monitor decontamination of polycarbonate cockpit canopies following exposure to chemical warfare agents and interest in the possibility of using SAW devices as gas sensors. Equations describing the effects of coating an SAW substrate with a polymeric film, tensions in a swollen static polymeric film attached to a SAW substrate, and the effects of compression film tension on SAW substrates were derived. These were combined to obtain an equation describing the frequency shift induced in an SAW substrate coated with a polymeric film induced by absorption of a vapor. The equation expressed the shift in frequency of the SAW substrate as a function of a term representing the mass loading effect of the film, a term representing swelling of the film induced by absorption of the vapor, and a term representing polymeric softening. Quartz substrate SAW crystals were prepared and coated with polycarbonate and polyimide films. The crystals coated with polycarbonate films were exposed to toluene (108883) vapor and the crystals coated with polyimide to methanol (67561). The extent of absorption and desorption and rates of absorption and desorption of toluene and methanol were determined. These, the weights and thicknesses of the films, and the shifts in frequency of the SAW substrates were measured. The extent of absorption and desorption on the polycarbonate coated crystals depended solely on the mass of the film. Absorption of the methanol vapor on the polyimide film coated crystal resulted in large negative shifts in SAW frequency. The magnitude of the shifts depended primarily on the mass of the film. No effect on the SAW response attributable to elastic tightening of the film was detected. The authors conclude that the changes in elastic properties of polymeric film coated SAW substrates appear to be due mostly to a mass loading effect of the films.