Sol-gel processes were developed to prepare nano porous silica materials. The obtained porous sol-gel silica (PSGS) materials have been used as constituent materials in designing optical fiber chemical sensors. A PSGS membrane coated on the surface of an optical fiber was used as a transducer for sensing humidity level in air. A PSGS membrane doped with an ammonia indicator dye has been coated on an optical fiber to sense ammonia in air. Both of the coating based sensors are reversible and fast response. In the tested range, relative humidity (RH) in air down to 3% can be detected with the PSGS coated fiber optic sensor. The fiber optic ammonia sensor with ammonia indicator doped PSGS coating can be used to sense ammonia in air down to sub-ppm level. PSGS has also been used as a constituent material in preparing porous silica optical fibers. The obtained porous optical fibers have been used to design optical fiber chemical sensors for sensing humidity, ammonia and volatile organic compounds. A CuCl2 doped PSGS fiber has been tested for sensing ammonia in a high temperature gas sample. Ammonia in the high temperature air gas diffuses into the PSGS fiber, reversibly reacts with CuCl2 doped in the PSGS fiber to form a complex. The formed complex was detected with fiber optic spectrometric method. This sensor can detect ammonia in a high temperature (450 °C) air gas stream down 0.3 ppm. Techniques of preparing PSGS, coating PSGS on an optical fiber, making a porous optical fiber with PSGS as a constituent material will be presented. Examples of optical fiber sensors using PSGS coatings and a PSGS fiber as transducers for gas sensing are presented.