When reacting ethylene-vinyl acetate copolymers (EVA) with sodium alcoholates in alcohol, the alcoholysis of EVA is accompanied by crosslinking reactions. Crosslinking can be detected by the increase in torque in a plastograph. This paper reports on investigations on the mechanism of the crosslinking reaction. Both crosslinking via C˭C bonds and formation of a thermally reversible network via polymer alcoholate structures can be found. If an ethylene-vinyl alcohol copolymer is obtained by complete saponification of EVA, however, only thermally reversible crosslinking can be demonstrated. This leads to the conclusion that irreversible crosslinking is established through a reaction of the acetate side chains. The reaction products of the low-molecular weight model substances, pentanol-3-acetate and sodium isopropylate in isopropanol, are analyzed by means of gas chromatography. These results suggest a partial Claisen condensation as a mechanism of the irreversible crosslinking process.