New isodethiaazacephems (+/-)-3, (+/-)-4, and (+/-)-10 as well as the 4-sulfonylated isodethiaazacepham (+/-)-5 were synthesized by chemical methods and found to possess biological activity against five pathogenic microorganisms in vitro. The mesylate and the triflate functionalities in (+/-)-3 and (+/-)-4, acting as effective leaving groups, enhanced remarkably the biological activity in comparison with the parent 3-hydroxyisodethiaazacephem (+/-)-10. The mode of action related to (+/-)-3 and (+/-)-4 can be explained by a [1,4]-elimination process.