We report on an unusual staining phenomenon of a diblock copolymer, poly(ferrocenylsilanes)-b-poly(dimethylsiloxanes) (PFS-b-PDMS). By a combination of transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis, it is found that when a thin section of diblock copolymer is exposed to ruthenium tetroxide vapor, the interfaces between the PFS and the PDMS microdomains are selectively stained. The effect appears to arise from a delicate competition between reactivity and diffusion. This observation broadens our knowledge of the staining behavior of diblock copolymers and provides a potential route to pattern ordered nanoring arrays using diblock copolymers as a template.