AbstractUV spectroscopic measurements to identify the products of photolysis in polystyrene solutions of CCl4and CHCl3as a function of the atmospheric oxygen concentration are conducted. In polystyrene chains CC bonds are formed, thus changing the absorption properties in the long wave length range. The concentration of induced CC bonds is higher in CCl4solution than in CHCl3solution. The effect of O2is observable for pressuresp≥ 2 · 10−1mmHg; it is of minor importance for the formation of CC bonds, however. On the low‐molecular‐weight side COCl2, Cl2, and HCl are the photolysis products in the presence of O2. Observable changes in the absorption of the solvent at 280 nm were attributed to the electronic n ← π* transition of the carbonyl groups of COCl2, whereas the newly created broad band in the UV spectrum of the solvent was attributed to the electronic1Π1u←1Σg+and3Π+ou←1Σg+transitions in Cl2. The first of these transitions is responsible for the dissociation of Cl2molecules. A hypothesis has been set forth that molecular oxygen affects the probability density of the electronic3Πou+←1Σg+transition. The concentration of O2has a substantial effect on the concentration of the COCl2photoproducts, which is best displayed by the molar absorption