The delustring phenomena, observed often during heat-treatment of cellulose diacetate (CDA) in hot water, were studied in connection with the molecular characteristics of CDA and the thermal motion of CDA molecules. The phenomena accompany with decreases in (1) density, (2) weight remaining, and (3) storage modulus, and disappeared by treatment of film under high pressure or by dipping it into good solvents for CA with relatively low degree of acetic acid content (AC). The delustring phenomena can be attributed to a change from uniformly packed state of CDA molecules in the original film to highly heterogeneously packed state by dissolution and/or swelling of CA molecules of low acetyl content and of low molecular weight and CA molecules having the groups of sulfonic ester and carboxylic acid as impurities during the course of heat-treatment in hot water. Such a change is closely associated with the microbrownian motion of main chains in the amorphous region. Water molecules act selectively on the CA molecules of low acetyl content and of low molecular weight as a dissolving, swelling, and plastisizing agent.