In situ melt polycondensation of L-lactic acid (LLA) in the presence of acidic silica sol (aSS) is proposed for the first time to prepare PLLA/SiO2 nanocomposites. The SiO2 nanoparticles were readily dispersed in LLA monomer, which has similar polarity and hydrophilicity to the silica sol medium. During the polycondensation process, both the matrix and the surface of SiO2 nanoparticles changed from high polarity/hydrophilicity to weak polarity/hydrophobicity due to simultaneous chain growth in the organic phase and chemical grafting on the particle surface. The chemical grafting provided steric stabilization and ensured satisfactory nano-scale dispersion in the final nanocomposites. The introduction Of SiO2 nanoparticles resulted in unchanged yield and better color. The molecular weight kept almost constant at low SiO2 content (< 8 wt%) but decreased at higher SiO2 content. The method is also characterized by commercially available and cheap starting material and environmentally benign process. It appears to be a promising approach for the preparation of PLLA/SiO2 nanocomposites. (c) 2007 Elsevier Ltd. All rights reserved.