Cellulose is the most abundant natural polymer on earth, and widely used in everyday-life products from building construction to clothing to paper. Fundamental understanding of molecular solutions of cellulose is the key to realize advanced technologies beyond cellulose fibers. It has been previously reported that certain ionic liquid and organic solvent mixtures dissolve cellulose. In this study, the tertiary phase diagram of cotton linter microcrystalline cellulose, 1-Ethyl-3-methylimidazolium acetate (EMIMAc), and dimethylformamide (DMF) mixtures has been determined using primarily optical cloud point method and small angle neutron scattering (SANS).  The mechanism of cellulose dissolution in tertiary mixture has been investigated by combining quasielastic and small angle neutron scattering (QENS and SANS).  As SANS data show that cellulose chains take Gaussian-like conformations in homogenous solutions, which exhibit characteristics of having an upper critical solution temperature, the dynamic signals predominantly from EMIMAc molecules indicate strong association of EMIMAc with cellulose in the dissolute state.  Data indicate that a molar ratio of EMIMAc to cellulose repeating unit greater or equal than 3 is necessary but not sufficient in forming one-phase homogeneous solutions.  Clustering of cellulose chains occurs at low EMIMAc/DMF or EMIMAc/cellulose ratio, or at low temperatures.  The mechanism of cellulose dissolution in tertiary mixture is discussed, and the potential applications of molecular cellulose are demonstrated.