Measurements of the thermal expansion coefficients (TECs) of chitin crystals in the lateral direction are reported. We investigated highly crystalline alpha chitin from the Paralithodes tendon and an anhydrous form of beta chitin from a Lamellibrachia tube from room temperature to 250 degreesC, using X-ray diffraction at selected temperatures in the heating process. For alpha chitin, the TECs of the a and b axes were alpha (a) = 6.0 x 10(-5) degreesC(-1) and alpha (b) = 5.7 x 10(-5) degreesC(-1), indicating an isotropic thermal expansion in the lateral direction. However, the anhydrous beta chitin exhibited an anisotropic thermal expansion in the lateral direction. The TEC of the a axis was constant at alpha (a) = 4.0 X 10(-5) degreesC(-1), but the TEC of the b axis increased linearly from room temperature to 250 degreesC, with alpha (b) = 3.0-14.6 x 10(-5) degreesC(-1). These differences in the lateral thermal expansion behaviors of the alpha chitin and the anhydrous beta chitin are due to their different intermolecular hydrogen bonding systems. (C) 2000 John Wiley & Sons, Inc.