A catalyst composition for use in the preparation of a polyester or copolyester is disclosed. The catalyst composition comprises the combination of (a) a titanium-based compound or zirconium-based compound, and (b) a lanthanide series compound or hafnium. Various titanium-based compounds and lanthanide-based compounds are described. Moreover, in some embodiments, the catalyst composition may further comprise a phosphate-forming compound. Another aspect of this invention is an improved method for preparing a polyester, based, for example, on the two-stage process of ester interchange and polycondensation. The method is carried out in the presence of the catalyst composition described above.