he electrophosphorescence (EPH) efficiency was found to increase by up to 6% for the devices based on a metallo-organic phosphor of iridium and 2% for those based on a metal-organic complex of platinum as an external magnetic field increased to 500 mT. Also, a difference in the field dependence of the EPH for these devices has been observed. We demonstrate that these experimental findings can principally be understood in terms of the Zeeman and hyperfine interactions of electrons located on phosphor molecules and holes located on the hole-transporting molecules, forming correlated electron-hole pairs in the emitting layer. The difference in the field behavior of the effect for the two phosphors results from their different molecular structures.