2-Hydroxyethylammonium formate acts as a task-specific ionic liquid (TSIL) for the Knoevenagel condensation of carbonyl compounds with rhodanine to afford arylalkylidene rhodanines under solvent-free conditions and in good-to-excellent yields. Additionally, compared with those in organic solvents, the yields obtained in the presence of our ionic liquid (IL) were significantly increased. The detailed mechanism of the catalytic effect of TSIL is also reported for the first time.