In the present research, a study was made on the photodegradability of blends of polystyrene (PS) or styrene acrylonitrile copolymers (SAN) with photodegradable polymers having vinyl ketone group at the side chain Styrene methyl vinyl ketone copolymer (SMVK) was selected as a photodegradable polymer with vinyl ketone group at the side chain. SMVK was synthesized by solution polymerization technique and blended with PS or SAN. These blends were then exposed to u.v. irradiation at various lengths of time, during which the variation of viscosity was tested to investigate the effect of the addition of SMVK to the blends on the photodegradability of PS or SAN as a funtion of time. It was turned out that the photodegradability of styrenic polymers increased by the addition of SMVK. While photodegradability of PS highly increased, that of SAN increased only slightly. As the AN content in SAN blends increased, the effect of SMVK on the photodegradability of the blends became lower. The photodegradability of these styrenlc blends with SMVK was considered to be affected by such factors as miscibility among components and photodegradability of components themselves. The above results were comfirmed by analyzing the viscosity, DSC, and FTIR data.