Solid-state 13C NMR spectra of graphite oxide GO. and its derivatives prompt us to propose a new structural model. The spectra of GO treated with KI and the course of the thermal decomposition of GO reveal the presence of epoxide groups, responsible for the oxidating nature of the material. GO is built of aromatic'islands' of variable size which have not been oxidized, and are separated from each other by aliphatic 6-membered rings containing C-OH, epoxide groups and double bonds. The carbon grid is nearly flat; a small degree of warping is caused by the carbons attached to OH groups, which are in a slightly distorted tetrahedral configuration.