In the green Knoevenagel reaction of benzaldehyde and malonic acid, the carbon–carbon formation reaction is catalyzed by a double Schiff base formed by benzaldehyde and ammonia. A mechanism for this green Knoevenagel has been proposed. A closed catalytic cycle explains why various ammonium salts, including ammonium bicarbonate, have been correctly reported as catalysts in Knoevenagel-like reactions. Various catalytically active intermediates from derivatives of benzaldehyde were isolated and characterized. Furthermore, these in situ formed double Schiff bases play another catalytic role in the consecutive decarboxylation reaction to various cinnamic acids.