Porphyrins consist of twenty-atom rings containing four nitrogen atoms and can be used as sensor to detect odours and gases. This thesis investigates whether or not porphyrins can be used as functional materials on grid gate devices.  Drops of PVC embedded porphyrins were deposited on the surface of a grid gate which is a Metal Oxide Semiconductor (MOS) capacitor. In order to detect the gas sensing properties of the porphyrins a light addressable method called Scanning Light Pulse Technique (SLPT) has been used. Drops of porphyrins were deposited with a stretched capillary tube (1 mm diameter). The MOS capacitor has been exposed to nitrogen atmosphere as reference environment, while the target gases were carbon monoxide (100 ppm) and ammonia (500 ppm). The result from the eight porphyrins is that one of them [Pt(II) TPP] has a response for both gases, ammonia induces a change in both the work function and surface resistance, while the carbon monoxide induces only a change in the surface resistance.