R-sulfoxides of the formulae ##STR1## wherein ##STR2## represents an usual penicillin or cephalosporin amide group and X is selected from the group consisting of hydrogen, hydroxy, an alkanoyloxy and the residue of a nucleophilic agent and their salts and esters are prepared by oxidation of the corresponding penicillins and cephalosporins to convert the sulfur atom in the ring to a sulfoxide group and separating and isolating the penicillin- or cephalosporin R-sulfoxide so formed from the reaction mixture or by acylation of the free amino group of the intermediate 6-aminopenicillanic acid R-sulfoxide and 7-aminocephalosporanic acid R-sulfoxide and derivatives. A preferred straight oxidation method is the photosensitized oxygenation method.