Modified poly(butylene terephthalate)-poly(tetramethylene glycol) multiblock copolymers were prepared by including either siloxane or silane units within the terephthalate hard blocks or the glycol soft blocks. These materials were characterized in terms of their dynamic-mechanical properties, glass transition temperatures T-g, and morphology las obtained from small-angle X-ray scattering measurements. One interesting result was the finding that putting siloxane units into the terephthalate hard blocks significantly decreased the storage modulus, whereas placing them into the glycol soft blocks somewhat increased it. Other interesting results were the observation that the T-g of the soft segments increased if the siloxane units were placed into the hard blocks but slightly decreased if they were placed into the soft blocks, while the hard-segment T-g was not observed at all. Some of these results could be explained in terms of the phase separation suggested by the X-ray scattering results.