The reactions of triplet oxygen atom with halomethanes as a potential fire extinguisher were studied by a discharge flow-chemiluminescence imaging method. The experiments were carried out under second order conditions. The bimolecular atom-molecule reaction rate constants were determined in terms of the initial rate method. The initial concentration of oxygen atom was also determined under second order rate law instead of the pseudo-first order conditions with $[O(^3P)]_0{ll}[sample]_0$. The second order conditions were more reliable than pseudo-first order conditions for the determinations of rate constants. The rate constants of the reactions $CF_3I;+;O(^3P)$, $CH_3PI;+'O(^3P)$, and $CHBrCl_2;+;O(^3P)$ were determined to be $5.0;{ imes};10^{-12}$ , $1.1;×;0^{-11}$ , and $1.9;{ imes};10^{-14}cm^3molecule^{-1}s^{-1}$, respectively.