We previously reported that amorphization was observed in a room temperature ionic liquid (RTIL)-6.7 mol% H2O mixture by simultaneous X-ray diffraction and differential scanning calorimetry measurements (Y. Imai, H. Abe, T. Goto, Y. Yoshimura, Y. Michishita, H. Matsumoto, Chem. Phys. 352 (2008) 224). Here, the RTIL is N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4]. In this study, the phase behavior of the [DEME][BF4]-x mol% (1 - y) H2O center dot yD(2)O mixtures is examined systematically by fixing the water concentration at around 7 mol%. Intriguingly, amorphization was completely suppressed and crystallization occurred even when a small amount of D2O was added. The crystallization temperature strongly depends on D substitutions; however, the crystal structure is not influenced by the D2O effect. (C) 2011 Elsevier B.V. All rights reserved.