Osmium tetroxide is a transparent very pale yellow crystalline solid. It is very reactive and extremely toxic. Especially for the eyes it is really dangerous. It usually is shipped and stored in glass ampoules, safely sealed and protected from the environment. The sample, shown above is from an old box, containing 10 ampoules, each ampoule holding a small amount of only 100 mg.

It is used in biology as a stain for microscopic samples. Its action is based on the oxidizing properties of the osmium peroxide, which is easily reduced to dark osmium dioxide or even osmium metal by organic matter, usually encountered in biological samples.