Surface acoustic wave (SAW) delay line (DL) oscillators offer many advantages as gas sensors. Such devices are fabricated on a piezoelectric (PE) substrate, whose propagation path is coated with a selectively sorbing film. The sorption produces a change in the SAW phase velocity, which can be detected as a frequency shift of the SAW oscillator frequency. Very often these sensors consist of a double DL oscillator circuitry which cancels the undesirable influences of the environment, e.g., temperature, pressure and so on. In practice one of the DLs serves as a reference. In this work we present the theoretical and experimental results obtained by many authors, concerning the SAW gas sensor response determination for three types of overlying thin films: insulating, conducting and semiconducting. The theoretical and experimental results are compared and discussed.