The capacity of coated interdigital capacitors (IDCs) is related to the SO 2 concentration in the ambient air. The amount of gas adsorbed on the coating determines the change of its dielectric properties. SO 2 molecules are adsorbed in a two-step process in two states. The temperature dependence is completely determined by the equilibrium between adsorption and desorption in the first step of this two-step process. The shape of the sensor characteristic is strongly non-linear. It is determined by the quotient of the number of available sites for SO 2 molecules on the sensor surface and the number of initially existing nuclei, which cause a formation of the molecules necessary for the transition between state 1 and state 2.