Polyesters are obtained by reacting at least one glycol having from 2 to 10 carbon atoms per molecule with at least one polycarboxylic acid or lower alkyl (C1-C4) ester thereof in the presence of a lanthanum-containing material other than lanthanum oxide as catalyst. The process is particularly suitable for the manufacture of polymeric ethylene glycol terephthalate from ethylene glycol and dimethyl terephthalate but blends of glycols, e.g. of ethylene glycol with diethylene glycol or pentaglycol (2,2-dimethyl-1,3-propanediol) may also be used as may the specified glycols alone. Mixtures of polycarboxylic acids, e.g. of terephthalic acid with isophthalic, sebacic, azelaic, suberic and adipic acid in the form of their lower alkyl esters may also be used as may the specified acids or their lower alkyl esters alone. Several forms of lanthanum may be used as catalyst, lanthanum in metallic form or a compound of lanthanum containing a radical of a weak acid or a radical of a volatile acid being particularly suitable. An additional esterification or transesterification catalyst such as materials containing antimony, germanium (e.g. GeO2), cerium (e.g. CeO2), calcium (e.g. Ca(OH)2), silicon (e.g. elemental silicon), cobalt, manganese, zinc or magnesium may also be present. The catalyst should preferably be appreciably soluble in the reaction medium either initially or during the course of the reaction and the amount of catalyst (calculated as the element) should preferably be between 0.001 mol. per cent and 0.07 mol. per cent of the polycarboxylic acid or its ester. In an example for the production of polymeric ethylene glycol terephthalate the monomer is first formed from dimethyl terephthalate and an excess of ethylene glycol in the presence of the lanthanum-containing catalyst the vapours distilling over being not allowed to exceed 70-75 DEG C. and suitable pot temperatures being 150-230 DEG C., e.g. 160-225 DEG C. The resulting monomer is polymerized in a tube provided with a side arm for distillation purposes and a capillary extending to the bottom of the tube through which pure dry nitrogen gas is introduced. A flask is connected to the side arm to collect the distillate, this flask being provided with a vacuum pump connection. Polymerization is effected by heating for several hours at 230-290 DEG C., e.g. at 265 DEG C., at 0.5 mm. Hg. pressure, and a table is given to show the results obtained when the polymerization is effected at 275 DEG C. using the formate, acetate, trifluoro-acetate, nitrate, phthalate, chloride, benzoate, benzene sulphonate, sulphate, and oxalate of lanthanum respectively as catalyst, the formate, trifluoroacetate, phthalate and benzoate being used in admixture with Sb2O3 as auxiliary catalyst and the chloride being used alone and in admixture with Sb2O3. The results obtained in a comparative example using lead oxide as catalyst are also tabulated. The results show that the lanthanum-containing catalysts generally give an improved reaction rate and a less-coloured product than is obtained when lead oxide is used and that the lanthanum salts of non-volatile strong acids (the oxalate, sulphate and benzene sulphonate) have only a low degree of catalytic activity. The Sb2O3 used as auxiliary catalyst is added after the first alcoholysis stage is completed and before the polymerization stage is begun. Other specified reactions in which the lanthanum-containing material may be used as catalyst are the ester-exchange reactions between (1) ethylene glycol and dimethyl sebacate; ethylene glycol and a 50-30 mixture of dimethyl terephthalate and dimethyl sebacate; (3) diethylene glycol and dimethyl terephthalate; (4) a 90-10 mixture of ethylene glycol and pentaglycol and dimethyl terephthalate; (5) ethylene glycol and isophthalic acid; and (6) ethylene glycol and terephthalic acid. Other specified esters which may be used are diethyl and dibutyl terephthalates. Specification 769,220 is referred to.ALSO:Esters of polycarboxylic acids are obtained by reacting at least one glycol having from 2 to 10 carbon atoms per molecule with at least one polycarboxylic acid or lower alkyl (C1-C4) ester thereof in the presence of a lanthanum-containing material other than lanthanum oxide as catalyst. The process is applicable to the production of monomeric and polymeric esters and is particularly suitable for the manufacture of monomeric and polymeric ethylene glycol terephthalate from ethylene glycol and dimethyl terephthalate but blends of glycols, e.g. of ethylene glycol with diethylene glycol or pentaglycol (2,2-dimethyl-1,3-propanediol) may also be used as may the specified glycols alone. Mixtures of polycarboxylic acids, e.g. of terephthalic acid with isophthalic, sebacic, azelaic, suberic and adipic acid in the form of their lower alkyl esters may also be used as may the specified acids or their lower alkyl esters alone. Several forms of lanthanum may be used as catalyst, lanthanum in metallic form or a compound of lanthanum containing a radical of a weak acid or a radical of a volatile acid being particularly suitable. An additional esterification or transesterification catalyst such as materials containing antimony, germanium (e.g. GeO2), cerium (e.g. CeO2), calcium (e.g. Ca(OH)2), silicon (e.g. elemental silicon), cobalt, manganese, zinc or magnesium may also be present. The catalyst should preferably be appreciably soluble in the reaction medium either initially or during the course of the reaction and the amount of catalyst (calculated as the element) should preferably be between 0.001 mol. per cent and 0.07 mol. per cent of the polycarboxylic acid or its ester. In an example for the production of monomeric ethylene glycol terephthalate from dimethyl terephthalate and an excess of ethylene glycol in the presence of the lanthanum-containing catalyst the vapours distilling over are not allowed to exceed 70-75 DEG C. and suitable pot temperatures are 150-230 DEG C., e.g. 160-225 DEG C. The resulting monomer may be polymerized in a tube provided with a side arm for distillation purposes and a capillary extending to the bottom of the tube through which pure dry nitrogen gas is introduced. A flask is connected to the side arm to collect the distillate, this flask being provided with a vacuum pump connection. Polymerization is effected by heating for several hours at 230-290 DEG C., e.g. at 265 DEG or 275 DEG C., at 0.5 mm. Hg pressure, and a table is given to show the results obtained when the polymerization is effected at 275 DEG C. using the formate, acetate, trifluoroacetate, nitrate, phthalate, chloride, benzoate, benzene sulphonate, sulphate, and oxalate of lanthanum respectively as catalyst, the formate, trifluoroacetate, phthalate and benzoate being used in admixture with Sb2O3 as auxiliary catalyst and the chloride being used alone and in admixture with Sb2O3. The results obtained in a comparative example using lead oxide as catalyst are also tabulated. The results show that the lanthanum-containing catalysts generally give an improved reaction rate and a less coloured product than is obtained when lead oxide is used and that the lanthanum salts of non-volatile strong acids (the oxalate, sulphate and benzene sulphonate) have only a low degree of catalytic activity. The Sb2O3 used as auxiliary catalyst is added after the first alcoholysis stage is completed and before the polymerization stage is begun. Other specified reactions in which the lanthanum-containing material may be used as catalyst are the ester-exchange reactions between (1) ethylene glycol and dimethyl sebacate, (2) ethylene glycol and a 50-30 mixture of dimethyl terephthalate and dimethyl sebacate, (3) diethylene glycol and dimethyl terephthalate, (4) a 90-10 mixture of ethylene glycol and pentaglycol and dimethyl terephthalate, (5) ethylene glycol and isophthalic acid, and (6) ethylene glycol and terephthalic acid. Other specified esters which may be used are diethyl and dibutyl terephthalates. Specification 769,220, [Group IV (a)], is referred to.