Butyraldehyde was aldolized with formaldehyde over a weakly basic anion-exchange resin catalyst in aqueous solvent in a batch reactor operating at atmospheric pressure and at temperatures 50-70 degreesC. The reaction mixture was a liquid-liquid-solid system, an emulsion, the phase equilibria of which were studied through chemical analysis of the organic and aqueous phase as well as of the mixed emulsion. Simplified rate equations were derived starting from molecular reaction mechanisms on the catalyst surface. A liquid-liquid reactor model for the fitting of the experimental results was developed on the basis of the rate equations and the phase equilibria, The model described very well the experimental data.