Diamines with neighbour nitrogen atoms have been used as base catalysts in the Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetoacetate. The catalytic results show that a good basic catalyst requires a combination of two factors: high proton affinity and the ability to return the proton to the oxoanion intermediate. Computational chemistry calculations show this by characterizing the reactants, products, and transition states and by calculating the activation energies of the different reaction steps. A diamine, di(3-methylpiperidine)methane (B), has been found with a higher catalytic activity than DMAN despite its lower proton affinity, demonstrating that not only the proton affinity, but also the steric ability to abstract the protons, are important in explaining the catalytic results. (c) 2006 Elsevier Inc. All rights reserved.