An azo dye derived from vinyl sulfone has been prepared having the following formula: A study has been made of the copolymerization of styrene with the above dye in the presence of benzoyl peroxide as an initiator. The colored polymers thus obtained preserve their shade even after repeated precipitation, because the dye is directly bound to the polymer chain. The polymerization has been studied in the temperature interval of 60–80°C, and the activation energy of the process has been determined. The effect of the amount of dye on the polymerization process has been investigated, and the proportion of dye chemically bound to the polymer has been determined spectrophotometrically. The effect of the dye on the molecular weight, correspondingly on [η] and the polydispersity of the colored polymers, has also been studied.