The crystalline structures of ethylene-propylene (EP) and ethylene-butene (EB) copolymers have been investigated using a combination of solid-state NAIR, WAXS, and DSC. In high-propylene (>20 mol %) EP copolymers, the a methylene exhibits different isotropic chemical shifts in the crystalline and amorphous regions. This indicates the presence of the methyl branch in the crystalline region and that the branch point has a trans conformation. As a result of the incorporation, on increasing branch content, the orthorhombic lattice expands in the EP copolymers. Then they exhibit a first-order transition from orthorhombic to a rotator (hexagonal) phase in which chains rotate about the chain axes. In the rotator crystals, about 5% of the carbons are methyl groups. At lower propylene comonomer contents, the crystal structure is orthorhombic, although with a larger area per chain and lower-field isotropic chemical shift value than observed for HDPE. It is proposed that the pattern of change of lattice constants and eventual transition into the rotator phase upon increasing branch content are closely related to the dynamics of the crystalline chains. In EB copolymers, in contrast, even at high comonomer concentrations, the crystal structure is still orthorhombic. The measured expansion coefficient of the rotator phase is much less than that of the same phase in n-alkanes, likely due to temperature-dependent branch incorporation.