In this study, we investigated the effects of phosphoric acid concentration on supramolecular structure of and enzymatic hydrolysis of the treated cellulose. A phase transition from cellulose swelling to cellulose dissolution was observed, when phosphoric acid concentration was greater than a critical value. On the basis of the above finding, we developed a simple procedure for producing a regenerated amorphous cellulose (RAC), precipitated from the dissolved homogeneous cellulose rather than from the swollen heterogeneous cellulose. Experimental Section