Uniform peroxopolytungstic acid (PTA) thin films with a high index were fabricated by dip-coating potassium ion-exchanged glass optical waveguides (OWGs) with aqueous and alcoholic PTA solutions. Dip-etching of the PTA film in water produced two 3 mm long tapered ends as couplers of the PTA/K+ composite OWGs, lending to adiabatic transition of the TE0 mode twice based on tapered velocity coupling theory between the potassium ion-exchanged layer and the PTA thin film. Such PTA/K+ composite OWGs can be applied to guided-wave absorption gas sensors by doping active dye molecules into the PTA films, which are expected to have a greater sensitivity than the popular evanescent absorption sensors. In this paper, PTA/K+ composite OWGs were used as a polarimetric interferometer for detection of formaldehyde toxic gas. A reversible interaction between PTA film and formaldehyde molecules, which caused a change in the phase difference between the zeroth order optical modes of transversal electric field and transversal magnetic field (TE0 and TM0 modes) in the PTA/K+ composite OWG. was observed, The sensing mechanism of the PTA film for formaldehyde gas is herein discussed.