Graphite oxide (GO) and its derivatives have been studied using C-13 and H-1 NMR. The C-13 NMR lines at 60, 70, and 130 ppm are assigned to C-OH, C-O-C, and >C=C< groups in the bulk of the material, respectively. The >C=C< double bonds are relatively stable, while C-OH groups may condense to form C-O-C (ether) linkages. There are at least two magnetically inequivalent C-OH sites, and the structure does not necessarily possess long-range order. Water molecules interact very strongly with the structure. The results reveal a number of new structural features.