In the present study, it has been unexpectedly found that solid-state polycondensation (SSP) can act as a facile method to prepare poly(ethylene terephthalate)/silica (PET/SiO2) nanocomposites with high molecular weight and an adjustable degree of branching or crosslinking. Fumed silica, with its surface silanol groups, seems to participate in some kind of reaction, probably esterification with the hydroxy end-groups of PET, during SSP, to act as a multifunctional chain extender. Differential scanning calorimetry and FT-IR spectroscopy reveal this ability of the silanol groups. The molecular weight increase depends on the used temperatures of SSP as well as on the amount of SiO2 added. As the amount of silica increases the rate of increase of the intrinsic viscosity slows because of the higher extent of branching. At 5 wt.-% SiO2 the extensive branching produces a crosslinked polymeric material. Such polyesters wit increased molecular weight and low silica content could be suitable for blown bottle production, while the high SiO2 content and adjustable branching or crosslinking could make the ideal high-melt-strength resins suitable for the preparation of low-density closed-shell foams.