Aldol condensation is a very important reaction in forming new C = C bond, and is an attractive way to extend the carbon chain of a molecule. In this research, diethylammonium acetate was used as an acid–base bifunctional catalyst to facilitate the formation of methacrolein via cross aldol condensation between formaldehyde and propionaldehyde. A conversion of 99.9% of propionaldehyde with a 97.4% selectivity to methacrolein (MAL) was achieved. Isotopic tracing experiments were used to propose a possible reaction mechanism. To verify the reactive pathway, we used isotopically labelled substrates. In addition, we attempted to trap the relevant reaction intermediate by decreasing the reaction temperature. [ABSTRACT FROM AUTHOR]