Macroscopic phase separation induced by thermooxidative reaction has been observed in a commercial styrene-block-isoprene-block-styrene (SIS) copolymer, Kraton 1107, upon exposure to atmospheric oxygen at elevated temperatures. Unlike styrene-block-butadiene-block-styrene (SBS) copolymers, there is essentially no gel formation (i.e., no cross-linking reaction) in the thermal oxidation of SIS copolymer. In the first phase separation, oxidative reaction causes the breakup of the SIS triblock chains into the SI blocks (containing polystyrene blocks and short isoprene segments) and low molar mass neat polyisoprene, forming an immiscible region with macrophase-separated domains. The continued decline of the molecular weight of polyisoprene and further detachment of isoprene fragments from the SI diblocks suppress the immiscibility region to a lower temperature, causing phase dissolution to occur at the reaction temperature. With progressive oxidation, the chain scission continues to occur while heavily oxidized lower molecular weight polyisoprene derivatives and oily byproducts are formed. Such chemical change in polyisoprene makes the system to be very unstable, thereby driving the macroscopic phase separation for a second time. It may be concluded that the cascading phase separation occurs due to chemical modification of the isoprene units of the SIS blocks as opposed to the cross-linking reaction induced macrophase separation in the SBS blocks.