In the gas phase selenium dioxide is present as dimers and other oligomeric species, at higher temperatures it is monomeric.

[6] The monomeric form adopts a bent structure very similar to that of sulfur dioxide with a bond length of 161 pm.

At higher concentrations the vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation.

For example, reaction with sodium hydroxide produces sodium selenite: Selenium dioxide is prepared by oxidation of selenium by burning in air or by reaction with nitric acid or hydrogen peroxide, but perhaps the most convenient preparation is by the dehydration of selenous acid.

It is only found at a small number of burning coal banks, where it forms around vents created from escaping gasses.

It is also renowned as a reagent for allylic oxidation,[11] a reaction that entails the following conversion This can be described more generally as; where R, R', R" may be alkyl or aryl substituents.

It is used in small quantities to counteract the colour due to iron impurities and so to create (apparently) colourless glass.