The weathering behavior of polystyrene (PS) and PS/montmorillonite nanocomposite with and without addition of ultraviolet (UV) absorber and/or antioxidant was investigated. Samples were exposed to UV radiation in the laboratory for periods of up to approximately 12 weeks. The samples were exposed for various irradiation intervals, and their tensile and impact strength was monitored through a factorial experimental design 2(4). The molecular weight, yellowness, and fracture surface were also monitored. The results generated by analysis of variance (ANOVA) Table showed that UV absorber and exposure time had the most important effects positive and negative, respectively. These results could be seen for neat PS and PS/MMT nanocomposite. The presence of antioxidant alone did not have any significant effect but when mixed with UV absorber presented a small synergism. These results corroborate with molecular weight, yellowing and fracture surface of tensile and impact samples. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 188-198, 2013