The homopolymers of styrene (S) and vinyl acetate (VA) and their copolymers were prepared in bulk by ultraviolet (UV)-radiation-initiated free-radical polymerization with azobisisobutyronitrile (AIBN) as an initiator. The reactivity ratios for these copolymerizations were determined by analyzing the monomer content in the copolymers by UV spectroscopy. The same method was extended to other copolymers of styrene such as styrene-methyl methacrylate and styrene-ethyl acrylate. A new analysis method was developed to measure reactivity ratios. For this purpose, UV light was used as a photochemical initiator and UV absorption spectroscopy was used for the determination of the instantaneous composition of copolymers. Nuclear magnetic resonance (NMR) was used to calculate percent conversion.