Process for the synthesis of a cyclic diester of an alpha-hydroxyacid, comprising the following steps: an alkalino-earth salt of the corresponding alpha-hydroxyacid is mixed with said alpha-hydroxyacid and water; the mixture is treated by evaporation-crystallization, so that a hydrate of the alkalino-earth salt of the linear dimer of the corresponding alpha-hydroxyacid precipitates; the hydrated salt is dehydrated to give the corresponding anhydrous salt; and the anhydrous salt is pyrolyzed, releasing the cyclic diester of the corresponding alpha-hydroxyacid and leaving the alkalino-earth hydroxide as solid residue.