We performed structural investigations of cellulose mixed with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) in the entire concentration range (0-100 mol %) by wide-angle X-ray scattering with the aid of quantum chemical calculations and 13C solid-state NMR spectroscopy. We particularly focused on a highly concentrated region (≥30 mol %), which has previously been overlooked. At concentrations of 15-30 mol %, a periodic peak corresponding to cellulose chain alignment emerged; this is associated with a lyotropic cholesteric liquid-crystalline phase. At concentrations of ≥30 mol %, the structure is transformed into ordered layers where OAc anions and Emim cations intercalate. This transformation is found to be driven by a change in the interaction between the IL anions and the OH groups of cellulose. At low concentrations, the anion mainly interacts with the OH group of cellulose in a 1:1 ratio, as previously reported; at high concentrations, the anions bridge the OH groups of two cellulose chains.