Abstract: A fiber optic chemical sensor and the method of making it are provided, such being useful in medical, biological and industrial applications. The sensor includes an optical fiber waveguide having a photocrosslinked polymeric gel at a portion thereof, which gel is crosslinked in place and undergoes substantial volume expansion. A dye is absorbed into the gel, the dye being responsive to different degrees depending on the presence and concentration of any one of particular parameters of a fluid being monitored, such parameters being pH, oxygen concentration, carbon dioxide concentration and the like.