The present invention provides an ionic liquid including a quaternary phosphonium salt represented by general formula (1): wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 each represent a linear alkyl group having 1 to 6 carbon atoms, a branched alkyl group having 3 to 6 carbon atoms, an alicyclic alkyl group having 3 to 6 carbon atoms, or an alkoxyalkyl group represented by --(CH.sub.2).sub.nO--R.sub.5 (wherein n is an integer of 1 to 6; and R.sub.5 represents a methyl group or an ethyl group); at least one of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is an alkoxyalkyl group; R.sub.1, R.sub.2, R.sub.3, and R.sub.4 may be the same or different; R.sub.1 and R.sub.2 together may form a ring; and X represents an anion. The present invention also provides a reaction solvent including the ionic liquid.