Iodine-bearing reactive materials are of interest for munitions aimed to defeat biological weapons because of strong biocidal properties of iodine-containing gas species. Ternary Mg center dot B center dot I-2 composite powders with 20 wt% of iodine were prepared by two-stage mechanical milling. A binary B center dot I-2 powder was prepared, chilled at dry ice temperature, and mixed with magnesium prior to the second milling stage. Materials with systematically varied Mg/B ratios were prepared. Resulting composite particles were near 30 A mu m in diameter. Iodine release and oxidation of the prepared powders were studied using thermogravimetric analysis in both inert and oxidizing environments. The most stable material was obtained with 33 wt% of Mg and 47 wt% of B. Up to 450 A degrees C (723 K), it showed only about 1 % mass loss. Iodine is lost in steps: the first step, around 100 A degrees C, is associated with release of unattached iodine. The second step, between 300 and 400 A degrees C, is associated with formation of MgB2. The majority of the iodine is lost above 600 A degrees C. The material did not age when stored in an inert gas; however, it became markedly less stable after storage in ambient air. Stepwise oxidation behavior was observed when the material was heated in an oxidizing gas. Oxidation started at a higher temperature than the initial weight loss caused by iodine release. Apparent activation energies, determined by model free processing for both iodine loss and oxidation, were near 100 kJ/mol for low-temperature iodine loss, and around 400-500 kJ/mol for high-temperature oxidation.