The isothermal sections of the Al-Cr-Mo ternary system at 1200 and 1000 °C were experimentally determined based on microstructure and phase constituents from the equilibrated alloys employing electron probe microanalysis, scanning electron microscopy, and x-ray diffraction. A new ternary compound phase named η with AlTi₃-type crystal structure covering a composition range with ~ 75 at.% Al was detected in these two investigated isothermal sections in the present work. Based on the experimental results and the published data of the three binary sub-systems, a thermodynamic description for the Al-Cr-Mo system was carried out using the CALPHAD (CALculation of PHAse Diagrams) method. The newly detected ternary phase η was modeled by a two-sublattice model of (Al)₃(Al,Cr,Mo)₁. A set of reliable thermodynamic parameters of the Al-Cr-Mo system was obtained, which are in satisfactory agreement with the experimental data. Based on the obtained thermodynamic parameters, much information related to the stable vertical sections, isothermal sections, liquidus projection, and miscibility gap of the bcc phase were predicted in the present work. The present work can provide essential experimental and thermodynamic data for the establishment of the Ni-Al-Cr-Mo based alloy database.