The importance of photooxidation in promoting formation of anhydride functional groups and thus promoting hydrolysis/biodegradation of polylactic acid and PLA nanocomposites were elucidated. PI-A-based nanocomposites were prepared by adding 5% wt filler content of sodium montmorillonite (ClNa), sodium montmorillonite partially exchanged with Fe(III) (ClFe); organically modified montmorillonite (Cl20A), unmodified sepiolite (SEP), and fumed silica (SiO2). The pure PLA and nanocomposites were UV-light irradiated in artificial accelerated conditions representative of solar irradiation (lambda > 300 nm) at 60 degrees C in air. The chemical modifications resulting from photooxidation were followed by IR and UV-visible spectroscopies. The infrared analyses of PLA photooxidation show the formation of a band at 1845 cm(-1) due to the formation of anhydrides. A photooxidation mechanism based on hydroperoxide decomposition is proposed. The mechanism proposed is confirmed by an increase in anhydride formation rate: the main responsible for this acceleration was identified as transition metals contained in the nanofillers as impurities and involved in the catalytic hydroperoxide decomposition.