A derivative of benzothioxanthene, N-(2-methacryloxy-ethyl)benzo[k,l]thioxanthene-3,4-dicarboximide (MBTD), was used as a fluorescent indicator for determination of water content in organic solvents. MBTD was photocopolymerization-immobilized on a glass surface. The prepared sensor system possesses relatively long lifetime, short response, and recovering time. The reversibility and reproducibility are also adequate for practical measurement.