Crystal structures in N,N-diethyl-N-methyl-N-2-methoxyethylammonium tetrafluoroborate, [DEME][BF4], are determined by X-ray diffraction method. In [DEME][BF4]-based mixtures, various kinds of crystal structures and superstructures are induced by additives, i.e., H2O, CH3OH, C2H5OH, and C6H6. Most of the crystal structures in the mixtures are related to that of pure [DEME][BF4], though unit cells in 1.1 and 6.1 mol % C6H6 are different from the pure one. Further, [DEME][BF4]-C6H6 mixtures require the two kinds of superstructures, where volume per [DEME][BF4] is contracted by a small amount of additives. Also, volume contraction and the coexistence of two kinds of superstructures are seen in 0.9 mol % H2O. The superstructures are derived from orientational or displacive modulations of [DEME][BF4] molecules. In spite of a small amount of additives, molecular interactions of [DEME][BF4] are influenced extensively.