Amorphous poly(ethylene terephthalate) (PET) and cellulose blends in film form were obtained by room temperature hydrolysis of PET/cellulose trifluoroacetate solution cast films. Evidence is presented indicating that the cellulose, or the water associated with it, nucleates the crystallization of the PET during differential scanning calorimetry (DSC) runs. At about the 50/50 composition, a phase inversion, from continuous PET to continuous cellulose appears. Hydrolysis and/or annealing in water at the boil yields a mixture of cellulose II and cellulose IV. The nature of the cellulose appears to be different in the case of the room temperature hydrolyzed structures. Hydrolysis appears to proceed more readily when the films are richer in the cellulose component.