Measurements of the thermal expansion coefficients (TECs) of cellulose crystals in the lateral direction are reported. Oriented films of highly crystalline cellulose I-beta and IIII were prepared and then investigated with X-ray diffraction at specific temperatures from room temperature to 250degreesC during the heating process. Cellulose I-beta underwent a transition into the high-temperature phase with the temperature increasing above 220-230degreesC; cellulose IIII, was transformed into cellulose I-beta when the sample was heated above 200degreesC. Therefore, the TECs of I-beta and IIII below 200degreesC were measured. For cellulose I-beta, the TEC of the a axis increased linearly from room temperature at alpha(a) = 4.3x10(-5) degreesC(-1) to 200degreesC at alpha(a) = 17.0x10(-5) degreesC(-1), but the TEC of the b axis was constant at alpha(b) = 0.5x10(-5) degreesC(-1). Like cellulose I-beta, cellulose IIII also showed an anisotropic thermal expansion in the lateral direction. The TECs of the a and b axes were alpha(a) = 7.6x10(-5) degreesC(-1) and alpha(b) = 0.8x10(-5) degreesC(-1). The anisotropic thermal expansion behaviors in the lateral direction for I-beta and IIII were closely related to the intermolecular hydrogen-bonding systems.