A bilayer resist system is demonstrated using chlorinated polymethylstyrene‐polydimethylsiloxane (CPMS/DMS) block copolymers. The copolymers are prepared using anionic polymerization techniques and are subsequently sensitized to electron beam, and 250–300 nm radiation by preferential chlorination of the polymethylstyrene block. The electron lithographic performance of a block copolymer containing 15.5 wt. % silicon, 0.58 chlorines per methylstyrene unit (totalM̄w=8.7×104) and polydispersity=1.3 was determined to exhibit a sensitivity (D0.5g)=0.9 μC/cm2, and contrast=1.3. The etch ratio compared to the HPR‐204 planarizing layer was 13.5 : 1 with an observed resolution range of 0.75 to 1.0 μm. These results derive from the intrinsic behavior of block copolymers, which provides for the synergistic combination of inherently distinct polymer species while avoiding macroscopic phase separation prevalent in homopolymer blends. The concept of utilizing block copolymers as resists is general and may be extended to positive tone resists as well.