In this work, the hydrogen bond formation ability of cation and anion of the ILs was determined by using the solvatochromic method and was found to be responsible for their catalytic activity in the Knoevenagel reactions. The enhancement in the hydrogen bond donor ability of the cations and the hydrogen bond acceptor ability of the anions can both lead to the increase in the catalytic activity of the ILs. From the proposed reaction mechanism, it was deduced that the cations of the ILs can form hydrogen bond with aldehyde to activate the C=O bond as electrophile, and the anions of the ILs can promote active methylene compound to produce carboanion as nucleophile, showing dual function activation effect in the catalytic processes. In addition, the optimized N,N-dimethylethanolamnium acetate as dual function activator for the Koevenagel reactions benefits from wide substrates tolerance, ambient reaction temperature, easy work-up procedure, and easy reuse of the IL. (C) 2012 Elsevier B.V. All rights reserved.