The collisional charge-transfer reaction between Ar + ( 2 P 3/2,1/2 ) and CO represents one of the most studied ion-molecule systems; many controversies persist among different studies, and the detailed quantum state-to-state charge-transfer dynamics remains unknown. Here, differential cross sections of the charge-transfer process between the spin-orbit ground Ar + ( 2 P 3/2 ) ion and CO are reported at three center-of-mass collision energies of 1.02, 0.72, and 0.40 eV using a home-built three-dimensional velocity-map imaging-based ion-molecule crossed beam setup. At all three collision energies, the direct energy resonant charge-transfer mechanism dominates the reaction, featuring predominantly forward scattering with the CO + product population peaking at the v ' = 6 and v ' = 7 vibrational levels. Only at the lowest collision energy of 0.40 eV is the significant backward peaked scattering product observed, with CO + populated from v ' = 4 to v ' = 8. There is no obvious evidence for the formation of CO + in excited electronic state A 2 Π + , in qualitative accord with previous theoretical predictions.