Reaction kinetics of thermooxidative degradations in a styrene-block-butadiene diblock (SB) copolymer has been investigated by taking into consideration oxidative chain scission and cross-linking reactions. The gel permeation chromatography (GPC) study revealed that the chain scission not only lowers the overall molecular weight of the copolymer but also drives macrophase separation between polystyrene-rich copolymer and polybutadiene segments. As evidenced in the gel content determination, the molecular weight increases rapidly when the cross-linking reaction becomes dominant. To describe the thermooxidative reaction in the SE copolymer, we propose a simplified reaction scheme in which macrophase separation occurs between polystyrene and polybutadiene segments. Numerical calculations showed that our kinetic model indeed captures the experimental trends such as initial reduction and subsequent increase in molecular weight. The location of concave minimum in the molecular weight vs time curve was found to depend on the competition between the rates of chain scission and of crosslinking reactions. The effect of kinetic parameters on the variation of molecular weight of the diblock has been demonstrated.