We present experimental results on post-tuning the dispersion of a two-dimensional photonic crystal waveguide made from Ge33As12Se55 chalcogenide glass by exploiting the material photosensitivity to near-bandgap light. The change in the refractive index and volume of the material in response to exposure to 633nm light resulted in a shift of more than 5nm in the resonant coupling wavelength between a tapered optical fiber and the modes of a W1 waveguide. This represents a first proof of principle demonstration of the photosensitive post-tuning of a planar photonic crystal device. (c) 2007 Optical Society of America.