The efficiency and wavelength sensitivity of photodegradation in polystyrene (PSt) in the presence of a flame retardant were studied. The effect of the concentration of a flame retardant on the degradation of PSt was also investigated. Four kinds of flame retardant containing bromine atoms in the molecule and having the same frame structure were used. Irradiation of monochromatic radiation was carried out using the Okazaki Large Spectrograph (OLS). The degradation of the PSt matrix was followed by UV-visible and FTIR spectroscopy for the chemical changes produced. Changes in the molecular weight and its distribution were estimated from the results of a gel permeation chromatography (GPC) measurement. In all additives tested, the photodegradation of PSt was accelerated by the addition of bromine-containing flame retardants. The most effective wavelength for the main-chain scission of PSt was 280 nm for the samples containing 1 phr of the flame retardant, while this wavelength shifts to 300 nm for the samples containing 2 phr of the flame retardant. A probable reaction scheme of the accelerated degradation in the presence of a flame retardant was proposed from the experimental results. The photostability of PSt containing a flame retardant appears to depend on chemical structures of the additive and compatibility of the additives to PSt. Further studies are required to solve the role of additives in polymer matrices.