The existence of polymeric varieties of cerium oxide was proposed so long ago as 18971, this contention being based, inter alia, upon the fact that the colour of the oxide is dependent, not only on its purity, but also upon the nature of the salt from which it is prepared by ignition. Although density figures quoted in standard text-books show obvious variation, I have been unable to trace any data correlating the density of cerium oxide prepared in different ways with the possibility of its existence in polymeric forms.