Blends of in situ polymerized PBT from cyclic oligomers (c-PBT) and PYB were prepared with varying compositions and compared with mechanical blends of conventional PBT and PVB. The materials were characterized by a variety of techniques including DSC. DMTA. DETA, FrIR, NMR and GPC. It was found that the in situ prepared blend of c-PBT/PYB has one glass transition temperature and shows evidence of miscibility. In contrast, the conventional blend of PBT/PYB shows incompatibility after blending. The cause of miscibility in the in situ prepared PBT/PYB blends is thought to be the formation of a graft copolymer. These results show that there are unique possibilities for in situ processing by combining polymerization of cyclic polyester oligomers with blending.