The influence of introducing 1,3,4-thiadiazole and phthalyl groups as a pending groups into the repeating unit of PVC has been studied on the bases of photostability measurements. The photostability of modified. polymers were studied by monitoring the carbonyl Index, polyene index and hydroxyl index, All results obtained indicated that PI, PII, PIII, PIV and PV gave lower growth rate of ICO, IPO and IOH with irradiation time with respect to unmodified PVC film (control). The following trends of photostability are obtained PI>PII>PIII>PIV>PV According to the experimental results several reaction mechanisms were proposed depending on the presence of 1,3,4-thiadiazole and phthalyl groups moieties in the polymer chain. The mode of chain scission was also investigated by measuring the variation of average viscosity molecular weight with irradiation time for modified PVC. The degree of deterioration (?) and the average number of chain scission (S) values were also calculated.