Membranes Structural properties have been studied using XRD and compare the different values of the average crystallite size by using correction equations. undoped ZnO and ZnO: Sb for (0.5- 2) % membranes has been dropped on glass and p-type porous silicon (PS) substrate at 400 °C to use it as a sensor for CO gas. The crystal growth of the films that were deposited on the PS was not uniform due to the nature of the PS surface and due to the breakage of the crystal structure of the membrane material. the sensitivity of membranes dropped on (PS) for CO gas was higher than on glass substrate.