As with ethylene-vinyl ester copolymers, crosslinking reactions are observed with ethylene-acrylic acid ester copolymers in the presence of sodium alcoholates. The reaction mechanism was examined by means of infrared spectroscopic characterization of the cross-linked products and kinetic measurements. In comparative investigations with methyl methacrylate polymers and sodium alcoholates, a reaction was not observed. This leads to the conclusion that the course of the reaction is determined by the hydrogen atom positioned alpha to the carbonyl group. When diethyl acetic acid ethyl ester (as the model substance) was reacted with sodium iso-propylate, only isopropanol could be found by means of gas chromatography. Ethanol could not be detected, as it should have been in the case of ester condensation. Therefore, the crosslinking can be attributed to an aldol addition.