The 1: 1 ion pair formation constants (K-IP(0)) of I -alkyl-3-methylimidazolium ([RMelm](+); R = butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe2IM](+)) ions with tetrafluoroborate ([BF4](-)), hexafluorophosphate ([PF6](-)), bis(trifluoromethanesulfonyl)amide ([NTf2](-)), and 2,4,6-trinitrophenolate (picrate, [Pic](-)) ions have been determined conductometrically in dichloromethane at 25 degrees C. The K-IP(0) determinations have also been made for symmetric tetraalkylammoniurn ions ([R4N](+); R = methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the K-IP(0) value of the [RMeIm](+) salt is almost independent of the length of the alkyl chain (R), whereas that of the [R4N]+ salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The K-IP(0) values of [BuMelm](+), [BuMe2IM](+), and [Et4N](+), which are similar in the van der Waals volume, are [Et4N](+), showing that the C2-H atom on the imidazolium in the order of [BuMelm](+)approximate to [BuMe2IM](+) ring makes an important contribution to the strong ion pair formation ability of [RMelm]+. For a given cation, the K-IP(0) value is generally smaller for the larger anion, i.e., [BF4](-) >= [PF6](-) : [NTf2](-) >= [Pic](-)for [Et4N](+) and [BuMe2IM](+), and [BF4](-) > [PF6](-) > [Pic](-) >= [NTf2](-) for [RMelm](+).