In recent years, much attention has been focused on research to prepare new generation of Poly (para – substituted styrene), and to study the irradiation, thermal and plasticization effects on stability of these new polymers. The effect of irradiation of plasticized Para- substituted polystyrene in solid films was studied at different intervals of irradiation times and at different percentages of added phthalates and terephthalate plasticizers. The degradation process was followed by UV-VIS, Fluorescence and FT-IR Spectroscopic techniques. to determine the type and amount of degradation that occurs during irradiation. The irradiated pure and plasticized polymers solid films showed an increase in the intensity of absorption band by the increase in irradiation time and increase in the amount of added plasticizers. On the other hand the intensity of fluorescence was decrease upon the increase in irradiation time and increase in the amount of blended phthalate and terephthalate plasticizers.  The analysis of the FT-IR spectra of the irradiated and non-irradiated samples, showed a noticeable formation of new bands, and their intensity was found to increase with the increase in irradiation time and also with the increase in the amount of added plasticizer. In addition, the observed increase in the intensities of the carbonyl and hydroxyl absorption regions of the FT-IR spectra, providing evidence for the photodegradation as well as photo-oxidation of polymeric chains. Some kinetics work was applied to the results on fluorescence intensity of the excimeric emission to evaluate the quenching efficiencies and photo quenching rate constant by applying Al Ani – Hawi equation. Electrophile substitution such as (Cl, and Br) in the para position of the polymer backbone should less stability towards UV – Irradiation, whereas, nucleophilic substitution such as ( - H, -CH₃, -OCH₃, -OC₂H₅, -C₆H₅, α – CH₃, α –OCH₃, Phenyl and – C (CH₃)₄ should higer stability towards irradiation of plasticization. Among the para-substituted polystyrene, Poly (4- fluorostyrene) should a very high stability towards irradiation and plasticization that all polymers used in these studies. It is even more stable than polystyrene, The mechanism of the photodegradation of these irradiated polymers was found to started from abstraction of α – hydrogen atom from the phenyl group followed by a random chain scission in the polymer backbone. A reposed mechanism for the photodegradation of para-substituted styrene in solid films and in solution was based on the decrease or increase in the functional groups the appears from the FT – IR spectra of irradiated solid films.