Bacillus cereus T and F48 1 O/72, B. stearorhermophilus ATCC 15 18, and Clostridium perfringens NCTC 8798 spore inactivation by 20, 50, and 80 mg chlorine dioxide (ClOz)/L at three pH values (4.5, 6.5, 8.5) were evaluated. C102 concentration, species and sporulation medium significantly (p < 0.001) affect spore sensitivity. B. cereus T sporulated in Modified G medium and B. srearothermophilus ATCC 15 18 are similarly C102 sensitive, and each is significantly more sensitive than Fortified Nutrient Agar-sporulated B. cereus T and F4810/ 72. The Bacillus spore populations are more C102 sensitive than C. perfringens spore populations. Treatment pH affects C. petfringens but not Bacillus inactivation. Spores lacking intact coats are significantly more CIOz sensitive than the strains with coats intact.