A process for making oligomers of polybutylene terephthalate (PBT) with desirable intrinsic viscosity and carboxylic end group concentrations includes combining 1.4-butane diol (BDO). terephthalic acid and a catalyst to from a mixture and heating the mixture until the clearing point is reached based on visual inspection. The PBT oligomers are used in a continuous process for making polybutylene terephthalate (PBT) resin.