A deuterated water effect in the room temperature ionic liquid (RTIL) N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF(4)], is observed in its crystal domain structures; crystallization temperature, T(c); crystal superstructures; and volume contractions. The above effect, seen in [DEME][BF(4)]-0.9 mol % H(2)O mixtures, is reduced in 1.0 mol % 0.75 H(2)O x 0.25 D(2)O and 0.9 mol % 0.5 H(2)O x 0.5 D(2)O mixtures and is completely suppressed in 1.3 mol % D(2)O mixtures. Interestingly, T(c) decreased systemically with D substitutions of water. In contrast to the crystal state, it was found that there is no difference between H(2)O and D(2)O mixtures in the liquid state, on the basis of X-ray diffraction patterns. At around 80-90 mol % H(2)O, the intermolecular correlation of [DEME][BF(4)] as a local structure changes to that of bulk water.