Process for preparing polyesters

A process for preparing a polyester which comprises reacting a di-lower alkyl ester of a difunctional carboxylic acid at least 90 mole% of which consists of terephthalic acid with a polymethylene glycol containing 2 to 10 carbon atoms at least 90 mole% of which consists of ethylene glycol in the presence of an ester-interchange reaction catalyst to form an ester-interchange reaction product, and then polycondensing it in the presence of a polycondensation catalyst, characterized in that a substantially uniform solution in ethylene glycol of (A) 20 to 150 millimole%, based on said di-lower alkyl ester of the difunctional carboxylic acid, of calcium acetate and (B) 2 to 25 millimole%, based on said di-lower alkyl ester, of cobalt acetate is used as the ester-interchange reaction catalyst and the ester-interchange reaction product is filtered through a filter having a size of 200 to 1,000 Tyler's mesh before carrying out the polycondensation of the ester-interchange reaction product.