The effect of UV light on polylactide/poly(methyl methacrylate) (PLA/PMMA) blends produced by melt-extrusion with a special emphasis on the peculiar influence of PLA stereocomplexes on the photochemical behavior of the blends is the focus of this paper. Stereocomplexable PLA have been prepared by melt-blending of high-molecular-weight poly(l-lactide) (PLLA), poly(d-lactide) (PDLA) and PMMA. The photochemical behavior of resulting PLA/PMMA blends was studied by irradiation under photooxidative conditions (λ > 300 nm, temperature of 70 °C and in the presence of oxygen). The chemical modifications induced by UV light irradiation were analyzed using infrared spectroscopy (IR) and size exclusion chromatography (SEC). Morphological changes were studied by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). It was shown that PDLA and PMMA don't affect the rate of photooxidation of PLLA. However, PLA stereocomplexes have a strong impact on the morphology of the blends during photochemical ageing.