Room-temperature ionic liquids (RTILs) are salts that remain as liquid at room temperatures. RTILs have attracted wide interest for their unique properties, which often differ from molecular solvents. Despite this, very little has been known about colligative properties in RTILs to this date. In this research, we studied redox potentials of electroactive ions in a series of RTILs to investigate if any colligative behavior can be recognized. Accordingly, we found that the redox potential of Fc(+)/Fc in a series of 1-alkyl-3-methylimidazolium (C(n)mim(+) where n = 2, 4, 6, and 8) exhibits colligative behavior, which depends strongly on the temperature and alkyl chain length of imidazolium, where the longer the alkyl chain of the cation is, the less ideal the colligative behavior of the RTIL.