Chlorosulfonyl-containing pyrazolone azo compounds (2a, 2b) have been prepared by reaction of the corresponding sodium sulfonate (1a, 1b) with thionyl chloride in the presence of a catalytic quantity of N,N-dimethylformamide in dry benzene. The effects of reaction temperature, time, catalyst and solvent amount on the yield of 2a and 2b were investigated. The results show that chlorination of la and lb under optimal conditions gives 2a and 2b in 95.5% and 99.2% yield respectively. The given method is facile and suitable for large-scale synthesis.