The inhibiting effect of phosphorus compounds on the polymerization to poly(butylene terephthalate) catalysed by titanium tetrabutylate was investigated by making use of model molecules. The inhibition, observed on both transesterification and direct esterification reactions, appears only in the presence of acidic OH groups in the phosphorus compounds, which permit the formation of complexes with the Ti atoms of the catalyst. The occurrence of such complexes was confirmed by infra-red spectroscopy.