Highly crystalline samples of cellulose triacetate I (CTA I) were prepared from highly crystalline algal cellulose by heterogeneous acetylation. X-ray diffraction of the prepared samples was carried out in a helium atmosphere at temperatures ranging from 20 to 250 degrees C. Changes in seven d-spacings were observed with increasing temperature due to thermal expansion of the CTA I crystals. Unit cell parameters at specific temperatures were determined from these d-spacings by the least squares method, and then thermal expansion coefficients (TECs) were calculated. The linear TECs of the a, b, and c axes were alpha(a) = 19.3 x 10(-5) degrees C-1, alpha(b) = 0.3 x 10(-5) degrees C-1 (T < 130 degrees C), alpha(b) = -2.5 x 10(-5) degrees C-1 (T > 130 degrees C), and alpha(c) = -1.9 x 10(-5) degrees C-1, respectively. The volume TEC was beta = 15.6 x 10(-5) degrees C-1, which is about 1.4 and 2.2 times greater than that of cellulose I-beta, and cellulose IIII, respectively. This large thermal expansion could occur because no hydrogen bonding exists in CTA I. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 517-523, 2009