A process for preparing at least one cyclic ester of formula: wherein R1, R2, R3 and R4 are each independently selected from hydrogen or a group selected from C 1-6 alkyl; C 2-6 alkenyl; C 6-10 aryl, C 1 - 10 alkylC 6-10 arylene; C 6-10 arylC 1-6 alkylene; or C 2-6 alkynyl; each group optionally substituted by one or more substituents selected from C 1-6 alkyl, C 1-6 alkoxy, by a catalyzed gas phase transesterification reaction of at least one alpha-hydroxy ester, characterized in that (a) a solid catalyst comprises at least one metal of Titanium (Ti) and Zirconium (Zr), this metal being chemically bound to a support through a metal-oxygen-silicon (M-O-Si) chemical bond and (b) the process comprises (i) vaporizing at least one alpha-hydroxy ester selected from the group consisting of C 1-6 alkyl esters of lactic acid, glycolic acid, 2-hydroxy butyric acid, 2-hydroxy-3-butenoic acid, 4-methoxy-2-hydroxy butanoic acid and mandelic acid or mixtures thereof into a gas phase of ester vapor or comprising ester vapor; (ii) passing the gas phase over said solid catalyst and (iii) recovering the cyclic esters from this gaseous product stream. wherein R1, R2, R3 and R4 are each independently selected from hydrogen or a group selected from C 1-6 alkyl; C 2-6 alkenyl; C 6-10 aryl, C 1 - 10 alkylC 6-10 arylene; C 6-10 arylC 1-6 alkylene; or C 2-6 alkynyl; each group optionally substituted by one or more substituents selected from C 1-6 alkyl, C 1-6 alkoxy, by a catalyzed gas phase transesterification reaction of at least one alpha-hydroxy ester, characterized in that (a) a solid catalyst comprises at least one metal of Titanium (Ti) and Zirconium (Zr), this metal being chemically bound to a support through a metal-oxygen-silicon (M-O-Si) chemical bond and (b) the process comprises (i) vaporizing at least one alpha-hydroxy ester selected from the group consisting of C 1-6 alkyl esters of lactic acid, glycolic acid, 2-hydroxy butyric acid, 2-hydroxy-3-butenoic acid, 4-methoxy-2-hydroxy butanoic acid and mandelic acid or mixtures thereof into a gas phase of ester vapor or comprising ester vapor; (ii) passing the gas phase over said solid catalyst and (iii) recovering the cyclic esters from this gaseous product stream. (a) a solid catalyst comprises at least one metal of Titanium (Ti) and Zirconium (Zr), this metal being chemically bound to a support through a metal-oxygen-silicon (M-O-Si) chemical bond and (b) the process comprises (i) vaporizing at least one alpha-hydroxy ester selected from the group consisting of C 1-6 alkyl esters of lactic acid, glycolic acid, 2-hydroxy butyric acid, 2-hydroxy-3-butenoic acid, 4-methoxy-2-hydroxy butanoic acid and mandelic acid or mixtures thereof into a gas phase of ester vapor or comprising ester vapor; (ii) passing the gas phase over said solid catalyst and (iii) recovering the cyclic esters from this gaseous product stream. (i) vaporizing at least one alpha-hydroxy ester selected from the group consisting of C 1-6 alkyl esters of lactic acid, glycolic acid, 2-hydroxy butyric acid, 2-hydroxy-3-butenoic acid, 4-methoxy-2-hydroxy butanoic acid and mandelic acid or mixtures thereof into a gas phase of ester vapor or comprising ester vapor; (ii) passing the gas phase over said solid catalyst and (iii) recovering the cyclic esters from this gaseous product stream.