AbstractThe addition of hydroxyl groups on double bonds during the synthesis of unsaturated polyesters is studied with the help of model molecules. These are diesters of unsaturated diacids (maleic and fumaric acids) or monoesters of unsaturated monoacids (crotonic, cinnamic andp‐cyanocinnamic acids) with 1,2‐propanediol. The addition reaction take place to appreciable extents only with bis(hydroxypropyl) maleate and fumarate. The reaction is catalyzed by strong acids such asp‐toluenesulfonic acid; it is reversible and favoured by high temperatures. A mechanism is disc