A systematic in-situ high-temperature study of Fe2+ -Mg distribution over the two octahedral sites, M1 and M2, in synthetic orthopyroxenes (En75Fs25, En61Fs39, En49Fs51, En25Fs75, and En17Fs83) was undertaken at 1000, 1100, 1200, and 1300 K by the single-crystal X-ray diffraction method. The reversal measurements of cation ordering states were made on samples En75Fs25, En61Fs39, En49Fs51, and En17Fs83. Because ofthe orthopyroxeneclinopyroxene phase transition observed at ~1255 K for En17Fs83, no site-occupancy determinations at 1300 K were made for En25Fs75 and En17Fs83. The site occupancies in the Mg-rich sample (Fs25En75) at 1300 K are anomalous because of the existence of a transitional structural state prior to a phase transition from the orthopyroxene to the protopyroxene phase. The asymmetry in the distribution of Fe and Mg over M1 and M2 sites is confirmed in the range 1000–1200 K, although it is much less pronounced than that determined from quenched natural samples at lower temperatures (873–1073 K). An analysis of the site-occupancy data based on the regular solution model of Saxena and Ghose (1971) yielded: ΔGexch = 7751 + 3.000T (±246) J/mol, WM1 = 10230 − 2.065T (±164) J/mol, and, WM2 = 14775 − 7.575T (±624) J/mol, where T is in kelvins. All three parameters are temperature-dependent, with ΔGexch increasing and WM1 and WM2 decreasing with increasing temperature. The WM1 – WM2 term increases with increasing temperature, in accord with the result of Shi et al. (1992) but in contrast to most previous studies. This result suggests that the atomic configurations around the cations in M1 and M2 sites become more dissimilar, and the asymmetry of the orthopyroxene solid solution increases at elevated temperatures (1000–1200 K). The sublattice (Shi et al., 1992) and the regular solution models (Saxena and Ghose, l97l), respectively, were used to derive the microscopic and macroscopic excess thermodynamic parameters (ΔGex, ΔHex, and ΔSex) and the activity-composition relations. The macroscopic excess parameters show positive deviations from ideal mixing. The ΔGex and ΔHex values agree reasonably well with those determined experimentally.