Please cite this article as: Rodriguez-Pastor, I., Ramos-Fernandez, G., Varela-Rizo, H., Terrones, M., Martin-Gullon, I., Towards the understanding of the graphene oxide structure: How to control the formation of humic-and fulviclike oxidized debris, Carbon (2014), doi: http://dx.doi.org/10.1016/j.carbon. 2014.12.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Abstract Former structural models of graphene oxide (GO) indicated that it consists of graphenelike sheets with oxygen groups, and no attention was paid to the resulting sheet size. We now provide evidence of the complex GO structure consisting of large and small GO sheets (or oxidized debris). Different oxidation reactions were studied. KMnO 4 derived GO consists of large sheets (20-30 wt. %), and oxidized debris deposits, which are formed by humic-and fulvic-like fragments. Large GO sheets contain oxygen groups, especially at the edges, such as carbonyl, lactone and carboxylic groups. Humic-like debris consists of an amorphous gel containing more oxygenated groups and trapped water molecules. The main desorbable fraction upon heating is the fulvic-like material, which contains oxygen groups and fragments with high edge/surface ratio. KClO 3 in HNO 3 or the Brodie method produces a highly oxidized material but at the flake level surface only; little oxidized debris and water contents are found. It is noteworthy that an efficient basal cutting of the graphitic planes in addition to an effective intercalation is caused by KMnO 4 , and the aid of NaNO 3 makes this process even more effective, thus yielding large monolayers of GO and a large amount of humic-and fulvic-like substances.