The nature of chlorine dioxide substitution for chlorine in the chlorination reaction sequence of pulp bleaching in not well understood. Multiple results may be obtained by simply altering the order of chlorine-chlorine dioxide addition and the amount of chlorine dioxide substitution. Kinetic curves for chlorine, chlorine dioxide, and various levels of chlorine dioxide substitution were generated. The effects of these various processes on delignification (kappa number reduction) and Adsorbable Organic Halogen (AOX) generation were investigated. More than half of the AOX was found to be generated within the first one minute of any reaction sequence. Similar portions of the delignification were also found to occur within the first one minute of chlorination. The work examines the assumption that chlorine and chlorine dioxide react with different portions of the lignin. Thus chlorine and chlorine dioxide compete for only a part of the lignin molecule. The order of chemical addition dictates which chemical most rapidly attacks the competitive region. The hypothesis that chlorine dioxide substitution is simply a competing/parallel reaction sequence was studied.