We present a new concept for optical sensing of hydrogen utilizing photonic crystals as gas transducers. Photonic crystals are periodic nanostructures, which allow for manipulation of the motion of photons. Inverse metal oxide opal structures e.g. can exhibit a so-called photonic band-gap. These structures act like mirrors for light energies in the band-gap regime. The energy and the width of the band-gap, and therefore the reflected color, are not only affected by structural properties but also by the permittivity difference of the metal oxide and the surrounding atmosphere. Based on this, gas sensors with optical read out can be build. We demonstrate the quantification of hydrogen concentration utilizing Tungsten oxide inverse opals, which were manufactured by a wet-chemical casting process. During the gas reaction the refractive index of the tungsten oxide changes and so the reflectance spectrum of the crystal is manipulated. This allows us to use the shift of the reflectance maximum as a sensor signal for contactless optical read-out. We also present a first model as well as simulation results on the photonic sensing mechanism.