Organic electrolyte solutions (i.e. mixtures containing an ionic liquid and a polar, molecular co-solvent) are highly versatile solvents for cellulose. However, the underlying solvent–solvent and solvent–solute interactions are not yet fully understood. Herein, mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate, the co-solvent 1,3-dimethyl-2-imidazolidinone, and cellulose are investigated using 1D and 2D NMR spectroscopy. The use of a triply-13C-labelled ionic liquid enhances the signal-to-noise ratio for 13C NMR spectroscopy, enabling changes in bonding interactions to be accurately pinpointed. Current observations reveal an additional degree of complexity regarding the distinct roles of cation, anion, and co-solvent toward maintaining cellulose solubility and phase stability. Unexpectedly, the interactions between the dialkylimidazolium ring C2−H substituent and cellulose become more pronounced at high temperatures, counteracted by a net weakening of acetate–cellulose interactions. Moreover, for mixtures that exhibit critical solution behavior, phase separation is accompanied by the apparent recombination of cation–anion pairs.