Abstract: A process for producing polybutylene terephthalate comprises contacting terephthalic acid or its salt or its ester and butylenes glycol, in the presence of a catalyst composition, which comprises, or is produced from, titanium or a titanium compound, a complexing agent, a phosphorus-containing ester, and optionally a solvent. The process can also comprise contacting terephthalic acid or its salt or its ester and a glycol, in the presence of a catalyst composition to produce an oligomer and contacting the oligomer with a phosphorus-containing ester in which the catalyst composition can be one that catalyzes esterification or transesterification or polycondensation.