Aldols are important intermediates in the production of polyols, which are raw material for lubricants, surface coatings, and synthetic resins. The activities and selectivities of gel-type and macroporous anion-exchange resin catalysts in the aldolization of butyraldehyde with formaldehyde in an aqueous environment were investigated at 60 °C. The experiments were carried out batchwise in a stirred glass reactor, and the reaction products were analyzed with high performance liquid chromatography. Two main products were observed, 2-ethyl-3-hydroxy-2-hydroxy-methylpropanal (trimethylolpropane aldol) and 2-ethyl-propenal (ethylacrolein). The product ratio was strongly dependent on the catalyst. The selectivity with respect to trimethylolpropane aldol varied within the range 2.7-7.3 for the catalysts investigated. Systematic kinetic experiments were carried out at 50-70 °C with the gel-type resin catalyst displaying the highest aldol selectivity. A kinetic model based on molecular mechanisms was able to describe the product distribution.