LiMn2O4 , which crystallizes in the spinel structure, is of great interest as a cathode material for Li-ion batteries due to the high natural abundance of manganese, low toxicity, fast ionic diffusion and excellent battery capabilities. The structure is closely related to that of LixMnyO2, a layered structure, which can also be described as a defective spinel structure. Here, we show that both LiMn2O4 and LixMnyO2 nanoparticles can be synthesized from a simple, low-temperature hydrothermal synthesis. By tuning a single synthesis parameter (Li-concentration) each of the 2 structures or mixtures hereof can be obtained. The products have been characterized by X-ray powder diffraction both in-house and at SPring-8. Rietveld refinement was carried out on the high-resolution data from the synchrotron source and detailed structural models for both the spinel and the layered phase were obtained, with focus on the size of the nanoparticles. Using a two-phase model, the Rietveld refinements showed a bimodal size distribution for the LiMn2O4, which, to our knowledge, has previously only been determined using SEM. Using the structural Rieveld models for LixMnyO2 and LiMn2O4, a comparison of the two structures was made, by refinement of the structural models of one phase using diffraction data from the other. It was shown that the LiMn2O4 phase can be described nearly as well using the layered model compared to the spinel.