The modality of the incorporation of silanol-terminated siloxanes into copolymers with butylene terephthalate were investigated and determined. Besides siloxanes, the other participants in this well-known copolymerization, called the catalyzed two-step transesterification/polycondensation reaction, are dimethyl terephthalate and 1,4-butanediol. The employed approach to this problem was to exactly examine all the possibilities of the reactions between particular pairs of reactants, as well as of single reactants. The final results were, firstly, that reactions between siloxanes and dimethyl terephthalate did not occur and, secondly, that the only possibility for silanol-terminated siloxanes to be incorporated into the copolymers was by means of their condensation with 1,4-butanediol. This means that siloxanes are incorporated into the copolymer by ether and not by ester linkages. (C) 2007 Elsevier Ltd. All rights reserved.