The structure and properties of homogeneous copolymers of ethylene and styrene (ES) and ethylene and octene (EO) were compared. Semicrystalline copolymers presented a broad spectrum of solid-state structures from highly crystalline, lamellar morphologies to the granular, fringed micellar morphology of low-crystallinity copolymers. The combined observations from density, thermal behavior, and morphology with primarily atomic force microscopy revealed that the crystalline phase depended on the amount of comonomer but was not strongly affected by whether the comonomer was styrene or octene. This was consistent with the exclusion of both comonomers from the crystal. However, ES and EO showed strong differences in the amorphous phase. ES had a much higher p-relaxation temperature than EO, which was attributed to restrictions on chain mobility imposed by the bulky phenyl side group. The deformation behavior of ES and EO exhibited the same trends with comonomer content, from necking and cold drawing typical of a semicrystalline thermoplastic to uniform drawing and high recovery characteristic of an elastomer. Aspects of deformation behavior that depended on crystallinity, such as yielding and cold drawing, were determined primarily by comonomer content. However, the difference in the p-relaxation temperature resulted in much higher strain hardening of ES than EO. This was particularly evident with low-crystallinity, elastomeric copolymers. A classification scheme for semicrystalline copolymers based on comonomer content, previously developed for EO, was remarkably applicable to ES.