Magnetic field effects on the emission from the B(3)Pi(0+u) state of Br-2 and Cl-2 and the B(3)Pi(0+) State of IBr and ICl have been investigated with the aid of excitation spectroscopy under various external magnetic fields up to 7.5 kG. The magnetic quenching was observed for all the investigated vibrational levels of Br-2 and the nu＇ = 2 level of IBr, whereas no detectable magnetic quenching was observed for the other levels of IBr and for all the investigated levels of Cl-2 and ICl. The magnetic shortening of emission lifetimes was also observed for Br-2. A linear relationship was found between the magnetic quenching efficiency and the magnetic shortening efficiency. From this it is derived that the former can be represented by the ratio of magnetically induced increment in the nonradiative decay rate to the intrinsic decay rate. This relation can explain the observed dependence of magnetic quenching of Br2 and IBr upon vibrational and rotational levels. The magnetic quenching efficiency observed for Br-2 and IBr was found to be linearly dependent upon the square of the magnetic field strength. This indicates that the observed magnetic quenching is caused by the direct mechanism.