In this work, the aim was to analyze the efficacy of hindered amine light stabilizers (HALS) in woodflour-polypropylene composites compatibilized with vinyltrimethoxysilane after moisture absorption and accelerated weathering. Moisture uptake of materials decreased with incorporation of silane due to diminished accessibility of water molecules to reactive regions. In dynamic mechanical experiments performed on wet samples, a marked reduction in the storage modulus in the glassy and rubbery zone was observed, since water has a plasticizing effect. After sample weathering, in a xenon-arc apparatus, the changes in chemical structure and physical properties after exposure were analyzed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, color measurement, flexural properties, and morphological analysis by scanning electron microscopy (SEM). The data showed that HALS maintain the brightness of the materials after aging and prevent sample whitening. They also reduced color loss after aging and the SEM micrographs revealed that they inhibit surface cracking during weathering. Although a slight decline in the mechanical properties was not completely avoided, the combination of the additives studied (UV absorbers and HALS) successfully prevented the deterioration of surface materials by UV radiation. (c) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2017-2026, 2011