Abstract: An installation for use in the discontinuous production of high molecular weight polyethylene terephthalate according to either a first process wherein the production starts by producing bis(2-hydroxyethyl)-terephthalate by transesterification from dimethylterephthalate and ethylene glycol, or according to a second process starting with the production of a precondensate by reacting terephthalic acid with ethylene glycol. The installation comprises substantially a first and a second reaction vessel and a connection between said vessels, and means whereby the flow of reaction product through said connection can be accurately controlled to permit all or only part of the contents of the first vessel to pass to the second vessel.