The emergence of Bacillus anthracis as a potential bioterrorism and biological warfare agent points to the need for safe, effective, and economical sporicides for infection prevention and control. This work examined the efficacy of iodine vapor decontamination technologies to inactivate a surrogate for B. anthracis, Bacillus thuringiensis spores on glass materials. 10(6)-10(7) colony-forming units of spores inoculated onto circular glass cover slips were treated with different concentrations of iodine vapor under various temperature and relative humidity. Only minimal spore killing activity was observed at low humidity. Higher humidity levels, as well as pre-hydration or post-hydration of the spores, increased the rate of inactivation as long as the contact between spores and iodine was maintained in a hydrated environment. Significant sporicidal activity of 3-log and 6-log spore reduction has been observed with 2.1 mg L-1 iodine vapor concentration at 90% relative humidity and 22 A degrees C, with 1 and 24 h of exposure, respectively. The results showed that the relative humidity of the environment is of major importance in regulating the rate at which the spores are inactivated by iodine. The results of this study may provide insight into the parameters of effective decontamination procedures for Bacillus spores using gaseous iodine.