Unit-cell parameters of synthetic (Mg,Fe)SiO 3 ortho- and clinopyroxenes were determined at regular intervals in the temperature range 293-1094 K using powder X-ray diffraction techniques. Volume thermal expansion coefficients calculated from these data show that orthopyroxenes expand faster than clinopyroxenes (i.e., alpha opx > alpha opx ), irrespective of their composition along the MgSiO 3 -FeSiO 3 join. For both ortho- and clinopyroxenes, alpha MgSiO3 exceeds alpha FeSiO3 . Axial thermal expansion coefficients calculated for each of the pyroxene phases studied here are a complex function of the changes in structure at high temperature. Thermodynamic calculations of the position of the phase boundary between MgSiO 3 ortho- and clinopyroxene show excellent agreement with the experimentally reversed boundary.