Poly(butylene succinate) (PBSu)/fumed silica nanocomposites were prepared in situ by condensation polymerization. TEM micrographs verified that the dispersion of the nanoparticles was homogeneous in the PBSu matrix, while some small agglomerates were also formed at a higher SiO2 content. C-13 NMR spectra affirmed that the hydroxyl end groups of PBSu could form covalent bonds with the surface silanol groups of SiO2. These interactions affected the molecular weight of the prepared nanocomposites. At low concentrations the SiO2 nanoparticles acted as chain extenders, increasing the molecular weight of PBSu, while at higher loadings they resulted in extended branching and crosslinking reactions, leading to gradually decreased molecular weights. Silica nanoparticles acted as nucleating agents, increasing the crystallization rate of PBSu. However, the degree of crystallinity was slightly reduced. Tensile strength and Young's modulus were significantly increased with increasing SiO2 content. The presence of the nanoparticles resulted in reduced enzymatic hydrolysis rates compared to pure PBSu, attributed to the smaller available organic surface, due to the incorporation of SiO2, and to the existence of branched and crosslinked macromolecules. Dynamic mechanical and rheological properties were also extensively studied. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2010-2024, 2011