Ruthenium tetroxide (RuO4) is a versatile agent for imparting mass density contrast to saturated hydrocarbon polymers. We have successfully employed RuO4 to examine the microdomain and crystallite morphologies of poly(ethylene-block-vinylcyclohexane) semicrystalline-glassy diblock copolymers via transmission electron microscopy (TEM). Ultrathin sections cut from blocks stained with RuO4 showed excellent contrast in TEM, revealing the individual polyethylene crystallites that formed within the preexisting block copolymer microdomains. The size and orientation of the crystallites, which previously could only be inferred from scattering data, are readily apparent in the micrographs. Moreover, such imaging directly reveals the lateral extent of the crystallites and the number of crystallites lying within the cross section of each microdomain.