AbstractBlock copolymers of butadiene and styrene containing 21 and 61 wt.‐% styrene as well as pure polybutadiene have been investigated with respect to the extent of crosslinking produced by the irradiation with60Co γ‐rays. The free radical yield at 77 K,G(R.), has also been measured. Addition of the styrene blocks to the butadiene chains causes a rapid decrease in bothG(X), theG‐value for crosslink formation, and the free radical yield in nearly the same proportion for both, butG(X) is about twice as high as expected if crosslinks arose only from free radical recombination reactions. The effect of the styrene segments in the block copolymers onG(X) is approximately the same as in random copolymers of butadiene and styrene up to 21 wt.‐% styrene comp