Sensors based on polycrystalline semiconductors convert a chemical signal, the pressure of a gas, into an electrical signal. Specifically, the conductivity of the material changes when it is exposed to a specific gas. It is well known that the electrical conductance of this type of sensors obeys a power-law response with the gas pressure. This is regularly derived resorting to the mass action law applied to proposed reactions at the surface of the semiconductor. In line with this approach, we evaluate two proposed models regularly used in the literature and we also develop a new approach based on the Wolkenstein ionosorption theory that model reactions involved in the sensing process. Our aim is to determine which of the models best describes the surface reactions that take place during gas sensing.