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This study aimed to investigate how an ultralow content of a molybdenum disulfide (MoS₂) two-dimensional particle affects the photodegradation mechanism of polystyrene (PS). Here, an accelerated weathering study was presented on neat polystyrene and its nanocomposites produced with 0.001, 0.002, 0.003 and 0.005 wt% of molybdenum disulfide (MoS₂) exposed for various irradiation intervals (up to 8 weeks). The polymer photo-transformations were monitored using size exclusion chromatography (SEC), infrared spectroscopy (FTIR), and UV-Vis spectroscopy. The FTIR and UV/Vis results indicate that the PS degradation mechanism was not altered by the presence of MoS₂ particles; however, the degradation reactions were slowed down at higher MoS₂ contents (>0.003%). The SEC results proved the stabilizer effect due to MoS₂ particles, where <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover accent="true"><mrow><mi>M</mi></mrow><mo>¯</mo></mover></mrow><mrow><mi mathvariant="normal">n</mi></mrow></msub></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover accent="true"><mrow><mi>M</mi></mrow><mo>¯</mo></mover></mrow><mrow><mi mathvariant="normal">w</mi></mrow></msub></mrow></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover accent="true"><mrow><mi>M</mi></mrow><mo>¯</mo></mover></mrow><mrow><mi mathvariant="normal">w</mi></mrow></msub></mrow></semantics></math></inline-formula>/<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover accent="true"><mrow><mi>M</mi></mrow><mo>¯</mo></mover></mrow><mrow><mi mathvariant="normal">n</mi></mrow></msub></mrow></semantics></math></inline-formula> values after 8 weeks were less modified when compared with the neat PS results. The MoS₂ acted as a UV stabilizer, and these two-dimensional particles acted by deactivating the free radicals generated by the PS matrix, even considering the low amount of the filler (<0.005 wt%).