Here, we have measured the glass transition temperature (T-g) of the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate-H2O mixed solutions as a function of H2O concentration (x mol% H2O). The glass-forming composition region was also determined. Contrary to the results of the quaternary ammonium type of ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate-H2O mixed solutions, we did not observed the multiple glass transition behaviour. We also measured the glassy Raman spectra of the solutions at T = 77 K. We find that the "nearly free" hydrogen bonded Raman band of water molecules in the aqueous [bmim][BF4] solution exists up to around x = 60 mol% H2O, even at T = 77K. (C) 2010 Elsevier Ltd. All rights reserved.