Abstract: This invention is directed to an improved process for producing a polyester oligomer. The process employs a pipeline reactor, having at least two stages, in which the degree of polymerization of an oligomeric feed material is raised from about 2-10 to about 2-40 and the ratio of carboxyl to hydroxyl end groups in the product is reduced. In a first stage of the process, a monomeric diol or polyol, such as ethylene glycol, is added to a melt of the oligomeric feed material. In a second stage of the process, the molecular weight of the oligomer is increased by removal of volatile reaction by-products, including water and polyol. The oligomer produced by the present invention may be used in making higher molecular weight polyesters.