Macroscopic phase separation has been observed in a commercial styrene-b-butadiene-b-styrene (SBS) triblock copolymer (Kraton 1102) upon exposure to atmospheric oxygen at elevated temperatures. A thermooxidative reaction occurs in the unsaturated bonds of polybutadiene segments which in turn causes the copolymer chains to break, resulting in macrophase separation between polystyrene-rich copolymer and polybutadiene segments. The macrophase separation driven by thermo oxidative degradation has been further confirmed in as-synthesized SBS triblock and also in as-synthesized SE diblock copolymers. While the first macrophase separation proceeds, the radicals that emerged from the thermooxidative chain breakage undergo cross-linking, thereby driving phase separation to occur for a second time. The occurrence of the second phase separation may be hereafter termed "cascading phase separation". Despite the differences in copolymer types and total molecular weights, the structure and growth dynamics of the three styrene-butadiene block copolymers are strikingly similar to each other. Of particular interest is the occurrence of the cascading macrophase separation in neat block copolymers driven by thermooxidative chain scission and cross-linking reactions.