The structure of TcO2 between 25 and 1000 degrees C has been determined in situ using X-ray powder diffraction methods and is found to remain monoclinic in space group P2(1)/c. Thermal expansion in TcO2 is highly anisotropic, with negative thermal expansion of the b axis observed above 700 degrees C. This is the result of an anomalous expansion along the a axis that is a consequence of weakening of the Tc-Tc bonds.