AbstractMelting temperatures of cellulose trinitrate (13.96% N) in mixtures with γ‐butyrolactone have been determined dilatometrically, and also by an optical method depending on depolarization of polarized light when transmitted through a semicrystalline sample. Results obtained by these two methods, and also by direct visual observation, are in good agreement. Whereas the dilatometric method is preferred for higher concentrations of diluent (volume fractionv1= 0.86 to 0.68) where the melting point is below 80°C., gas generation by decomposition necessitated abandonment of this method at the higher melting temperatures for lower diluent concentrations. Adoption of the optical method permitted determination of melting temperatures as high as 116°C. (v1= 0.60). The results have been treated according to the thermodynamic theory of melting of polymers. While the results do not extend far enough to allow definite assignment of the melting pointT m0of the pure polymer, they indicate that it is at least 1000°K. The heat of fusion per structural unit is low—not over 1500 cal. per repeating unit, or 5 cal./g. The low value of the entropy of fusion—less than 1.5 cal./deg./mole of units–indicates limited flexibility of the chains in the liquid state. The high melting point of cellulose trinitrate is a consequence of its low entropy of fusion, and cannot be attributed to large intermolecular attractions i