Slow photons in photonic crystals are shown to optically amplify the photoactivity of anatase TiO2 in an inverse opal structure. An enhancement in TiO2 absorption as a result of slow photons leads to a larger population of electronhole pairs and faster degradation of organic molecules. A remarkable twofold enhancement is achieved (see figure) when the energy of the slow photons is optimized with respect to the absorption edge of anatase.