Thermodynamic parameters have been calibrated for a geobarometer suitable for use on eclogitic mantle xenoliths. The barometer is based on the incorporation of tetrahedrally coordinated aluminum in clinopyroxene coexisting with garnet and has been calibrated using the results of piston cylinder and multi-anvil experiments performed between pressures of 3 and 7 GPa and temperatures from 1,200 to 1,550 °C. Starting materials were hydrous and anhydrous synthetic mixtures of basaltic bulk compositions that yielded homogeneous bimineralic garnet–clinopyroxene phase assemblages. The experimental data set was expanded by employing results from previous experimental studies conducted in eclogitic systems, which widened the range of applicable conditions and compositions. The calibration reproduces experimental pressures of bimineralic eclogite assemblages, in addition to SiO2-saturated and kyanite-bearing eclogites, to within 0.4 GPa at the 95 % confidence interval. The barometer was then used to examine equilibration pressures recorded by natural mantle eclogites from various xenolith locations covering a wide pressure, temperature, and compositional range.