The Knoevenagel condensation of diethylmalonate and various aldehydes catalyzed by proline in a common ionic liquid was achieved. Mild reaction conditions, enhanced rates, improved yields, recyclability of ionic liquids containing proline, and reagents' reactivity which is different from that in conventional organic solvents are the remarkable features observed in ionic liquids. The ionic liquid containing catalyst was recycled in four subsequent runs with no decrease in activity. In addition, the results of calculations with the Gaussian 98 suite of program are in good accordance with the experimental outcomes. (c) 2006 Elsevier B.V. All rights reserved.