A series of alkylimidazolium salts containing dimethyl phosphate, methyl methylphosphonate, or methyl phosphonate prepared by a facile, one-pot procedure were obtained as room temperature ionic liquids, which have the potential to solubilize cellulose under mild conditions. Especially, N-ethyl-N′-methylimidazolium methylphosphonate enables the preparation of 10 wt% cellulose solution by keeping it at 45 °C for 30 min with stirring and rendered soluble 2–4 wt% cellulose without pretreatments and heating.