Ultrathin iron oxide particles are of high interest due to their large surface areas and unusual physical and chemical properties. Previous works have shown that single sheet iron oxides (SSI) can be formed via delamination of oxidized layered iron (hydr)oxides (green rusts, GR) interlayered by dodecanoate. However, there is considerable uncertainty on the true structure of both starting material and final products, and the reaction pathway. In this work, we describe a robust method for SSI synthesis and provide detailed structural characterization of the initial, intermediate and final phases to decipher the reaction mechanism. The SSI product has the formula FeO_0.82(OH)_1.38·0.7H₂O, and consists of platelets with a height of 1 nm and lateral dimensions of 20 to 100 nm as observed by Atomic Force Microscopy and Transmission Electron Microscopy. Mössbauer spectroscopy from 300 to 9 K shows that SSI is distinct from goethite, ferrihydrite and feroxyhite. Pair distribution function (PDF) analysis of high energy X-ray scattering data reveals that SSI has two distinct nearest neighbor Fe–Fe distances in contrast to the single distance in the parent Fe^II-Fe^III (hydr)oxide composed of entirely edge-sharing octahedra. Modeling of the SSI PDF data indicates that oxidation of Fe^II to Fe^III of dodecanoate-intercalated green rust results in displacements of Fe atoms perpendicular from the parent iron (hydr)oxide layer, forming a material that consists of iron polyhedra linked by both corner- and edge-sharing. This model which is different from the previously published model, matches the measured SSI thickness and electron diffraction pattern. This elucidated reaction pathway confirms that the dodecanoate interlayers in GR hinders Fe polymerization across interlayers and thus restrict chemical transformations to largely two-dimensional space. The increase of single- and double- coordinated O/OH groups in the SSI compared with the parent GRs is expected to give a high reactivity of SSI as surface complexation sorbents.