Treatment of 9, 2'-diacetylmidecamycin (2) with DMSO and acetic anhydride afforded 3"-methylthiomethyl derivative (3) preferably in the presence of pyridine. Reaction of midecamycin (1) with DMSO and acetic anhydride gave 2'-acetyl-9-dehydro-3"-methylthiomethyl derivative (9) indicating that the three hydroxyl groups reacted in a different way to the reagent. When compound 2 was reacted with DMSO and acetic anhydride in the presence of CCl4, 3"-acetoxymethyl derivative (13) was a major product, which was formed via 3 through the PUMMERER rearrangement. The structures of 3, 9 and 13 were confirmed by examining NMR and mass spectra of these compounds and their deuterio analogues. They showed antimicrobial spectra similar to 1 but superior in vivo activity.