The phase relation in the Al rich corner of the Al-Cr-Mg system was re-investigated at 400, 450, and 500 °C using diffusion couples, quenching experiments, and differential thermal analysis techniques. The homogeneity range of the Al18Cr2Mg3 phase was determined to be Al-(8.7 ± 0.6) at.% Cr-(13.0 ± 2) at.% Mg. It was also confirmed that there is no ternary compound other than Al18Cr2Mg3 in the Al-Al7Mg3-Cr system. A thermodynamic modeling for the binary Al-Cr and ternary Al-Cr-Mg systems was carried out based on the critically evaluated experimental data and new phase equilibria data from the present study.