The electromechanical response of a chemical gas sensor based on a TFBAR (Thin Film Bulk Acoustic Resonator) structure coated with a sensitive polymer layer was simulated with a FEM (Finite Elements Method) software (COMSOL Multiphysics®). The principle of sensing is based on the change of the mechanical properties of the polymer due to the gas adsorption. This will cause an additional mechanical load inducing a shift in the electrical input admittance curve of the resonator which decreases the resonance frequency of the fundamental thickness mode. This work presents a comparison between two adsorption models describing non swelling and swelling mechanisms by using FEM simulation. The presented structure is a piezoelectric capacitance Al/ZnO/Al with thicknesses of 0.2 µm /6 µm /0.2µm respectively above which a thin film (0.4 µm) of CH2Cl2-sensitive Polyisobutylene (PIB) polymer is coated. The structure vibrates at a resonance frequency of 491.2 MHz with a maximum mechanical displacement of 2.3 nm in the fundamental thickness mode. The FEM simulation has shown a sensor sensitivity of 3 Hz/ppm. The comparison between the two models shows that the model which considerates the swelling effect is more appropriate for describing the real sensitivity of the device.