Ruthenium tetroxide solution was prepared in situ by oxidation of ruthenium compounds at lower oxidation states with an excess of sodium periodate. The solution was able to stain saturated polymer POM in the vapor phase and it remained effective for up to 2 weeks. The melting behavior of POM samples stained for different lengths of time was studied with DSC and the staining process was analyzed. The results indicated that ruthenium tetroxide affected both the amorphous region and the crystals. During the early stage of the staining process, the tetroxide reacted preferentially with the amorphous material near the specimen surface. Much of this affected material was washed away after the rinsing process, thus resulting in an apparent increase in crystallinity. Prolonged staining would cause more crystals to degrade; hence, the crystallinity would drop. A two-step vapor staining technique was developed to improve the contrast between the amorphous and crystalline regions of ultrathin POM sections. The lamellar structure of POM spherulites was revealed and examined under TEM. The results showed that POM spherulites possess a category 2 structure and the thickness of the lamellae is about 7 nm.