The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements and optical cycling tests. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the OW6+O and W6+O bonds, respectively. When the Pd/a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to OW5+O and W5+O bonds appear at 330 and 450 cm−1, respectively. Optical cycling tests confirm that the Pd/a-WO3 films can not be bleached without oxygen. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen (18O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.