We present the optical characterization of an all-dielectric photonic crystal (PC)-based guided resonance filter sensitive to index-of-refraction changes in aqueous solutions. Spectral peak width was found to be 9.8 and 4.4 nm around 816 nm (water media), corresponding to a quality factor Q of 83 and 181, respectively. A spectral shift of peak wavelength with index change of 130 nm/RIU was observed for bulk fluid experiments. Measured peak shift (Δλ=0.2nm) corresponds to a detectable index change Δn=1.5×10-3.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.