Butyraldehyde was aldolized with formaldehyde at 40-80 degrees C over an anion-exchange resin catalyst in aqueous and methanolic solutions. The reaction mixture was analyzed with gas and liquid chromatography as well as nuclear magnetic resonance (NMR) spectroscopy. Two main products were detected: 2-ethyl-3-hydroxy-2-hydroxymethylpropanal (trimethylolpropane aldol) and 2-ethylpropenal (ethylacrolein). The molar ratio of the main products was dependent on the formaldehyde-to-butyraldehyde ratio and the solvent composition but independent of the temperature. The yield of 2-ethyl-3-hydroxy-2-hydroximethylpropanal increased with increasing formaldehyde-to-butyraldehyde ratio and with increasing water-to-methanol content of the reaction mixture. Besides the main reactions, self-condensation and acetalization of butyraldehyde were detected as side reactions of minor importance. The self-condensation of butyraldehyde was suppressed in the presence of formaldehyde and the acetalization of butyraldehyde was minimized by the use of water as reaction solvent. Rate equations were derived starting from molecular reaction mechanisms and adsorption of formaldehyde on the catalyst structure. It turned out that the rate equations can predict the distribution of the main products.