An optical fiber has been developed that can be used as a chemical gas sensor. Fabrication of the optical fiber produces a fiber that has a solid core with a porous cladding. The porous cladding region is made from a spinodally phase separable glass where the secondary phase is removed through dilute acid leaching. A non-phase separable glass composition is used for the core region. The properties of the phase separable glass are dependent on the processing conditions and the thermal history of the glass after the porosity has been achieved. Investigation of how processing conditions affected the pore structure was conducted to determine what pore characteristics are achievable for the glass composition used. Phase separation temperature, removal of silica gel deposited in the pores, and the post fabrication heat treating were used as experimental processing conditions. A maximum useable average pore size of approximately 29 nm was achieved. Maximum pore volume in the experimental groups was 0.4399 cc/g. Most heat treatments of the porous glass caused consolidation of the pore structure, with some conditions producing pore coarsening. / Master of Science