A highly sensitive optical waveguide sensor was developed for gaseous ammonia detection. It was based on an Ag+/K+ composite ion doped glass optical waveguide (OWG). A thin sensing film, consisting of a polymer matrix and a dye, was coated onto the optical waveguide (OWG) surface. Its response was reversible and quick (< 2.5 min), and detection limit was less than 100 ppb.