Thionine, an organic fluorescence dye, has been encaged in silica solid matrix via sol-gel method and the spectroscopic properties of the encaged fluorescent molecules have been investigated. Experimental results show that fluorescence intensity of the encaged thionine molecules is dramatically increased and emission wavelength shifts toward shorter wavelength, compared to those dissolved in water, anhydrous alcohol and silica sol. The encaged organic dye appears to be more photochemically stable that makes the encaged material suitable for solid-state optical sensor applications. (C) 2003 Elsevier B.V. All rights reserved.