TUNGSTEN TRIOXIDE FILMS HAVE BEEN PREPARED BY SOL – GEL TECHNIQUE, USING P-PTA AS A PRECURSOR. THE X-RAY DIFFRACTOMETERY, (XRD) PATTERNS INDICATED THAT THE CRYSTALLIZATION OF TUNGSTEN OXIDE OCCURS AT TEMPERATURES HIGHER THAN 500OC. THE ATOMIC FORCE MICROSCOPY (AFM) IMAGES SHOWED THAT THE GRAIN SIZE OF THE ANNEALED FILMS AT TEMPERATURES ABOUT 600OC IS SMALLER THAN 100 NANOMETERS. THE SENSING PROPERTIES OF THESE LAYERS TO CO, H2 AND CH4 GASSES IN DIFFERENT TEMPERATURES WERE MEASURED BY DYNAMIC METHODS .DUE TO SMALL GRAIN SIZE; THE SAMPLES EXHIBIT GOOD SENSITIVITY, RESPONSE TIME AND SELECTIVITY TO HYDROGEN AS COMPARED WITH THE OTHER GASES.