A simple method for synthesis of S-sulfoxides of penicillanic acid and 6 alpha-bromo- and 6 alpha-chloropenicillanic acids is described. The S-sulfoxides were synthesized by the respective oxidation with 30% hydrogen peroxide at 0 degrees C in the absence of solvents. 1 mol of penicillanic acid and 1.5 to 2 mol of 6 alpha-bromo- or 6 alpha-chloropenicillanic acid were required. The yields of S-sulfoxides of 6 alpha-bromo- and 6 alpha-chloropenicillanic acids amounted to 55 and 50%, respectively, based on 6-aminopenicillanic acid. The yield of penicillanic acid S-sulfoxide was 80% based on penicillanic acid. The purity of the compounds was more than 95%.