Penicillin sulfoxides are converted to 3-halo-3-methyl-cepham-4-carboxylic acid esters and amides or the corresponding cephem derivatives by heating the penicillin sulfoxide precursor in a polyhaloalkane solvent to a temperature between 50.degree. to about 150.degree. C. in the presence of an equimolar amount of a neutral or basic catalyst, respectively. The catalysts employed are bases such as pyridine, picoline, lutidine, quinoline, isoquinoline, dimethylaniline or a quaternary ammonium salt. The 3-halo-cepham intermediates produced with a quaternary ammonium catalyst are dehydrohalogenated to afford known cephem derivatives or converted to the free 4-carboxylic acid and the alkali metal, alkaline earth metal or ammonium salts thereof to afford novel antibacterial agents.