In the present study, the effect of silica nanoparticles, on the solid state polycondensation (SSP) kinetics of poly(ethylene terephthalate) (PET) is thoroughly investigated. At silica concentrations less than 1wt% and reaction temperatures between 200 and 230°C higher intrinsic viscosity (IV) values were measured, compared to neat PET at all reaction times. However, with 1wt% of nanosilica (n-SiO2), the IV increase of the nanocomposites was similar to that of neat PET and a further increase to 5wt% n-SiO2 resulted in significantly lower IV values. A simple kinetic model was also employed to predict the time evolution of IV, as well as the carboxyl and hydroxyl content during SSP. The kinetic parameters of the transesterification and esterification reactions were estimated at different temperatures with or without the addition of n-SiO2. The activation energies of both reactions were determined together with the concentration of inactive end-groups. From the experimental measurements and the theoretical simulation results it was proved that n-SiO2 in small amounts (less than 1wt%) enhances both the esterification and transesterification reactions at all studied temperatures acting as a co-catalyst. However, as the amount of nanosilica increases a number of inactive hydroxyl groups were estimated corresponding to participation of these groups in side reactions with the nanosilica particles. These side reactions lead initially to branched PET chains and eventually (5wt% n-SiO2 concentration) to crosslinked structures.