The mechanism of pink colouration with selenium in glass has been studied. When selenium-containing glasses are melted under reducing conditions, loss of selenium from the melt is decreased but a dark brown colour results due to the iron-selenide-oxide complex formation. Even a small amount of Ce02 in the batch oxidezes selenium and bleaches the pink colour; large concentrations of arsenic oxide in the batch also have the same effect. Oxidizing conditions in the melt do not produce any larger selenium loss from the melt than with "neutral" melting conditions. On electron microscope examination, pink glasses coloured with selenium revealed spherical particles.