N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4], and H2O mixtures were investigated in optical properties by UV-visible spectroscopy. The concentration region of H2O in the mixtures was 0-100 mol%. At room temperature, the mixtures of H2O exhibit absorption peaks at around 280 nm and 750 nm, which depend on H2O concentrations. At 90-95 mol% H2O, the optical transmittance decreased over the whole region of UV-visible light. In X-ray diffraction, correlation over the medium-range such as network-forming materials develops at 90-95 mol%. Also, in quenched glass, low glass transition temperature appeared at these H2O concentrations. The specific glass is induced by different molecular aggregation in liquid. Therefore, an optical anomaly is connected with the specific water network over the medium-range order. (C) 2011 Elsevier B.V. All rights reserved.