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Experimental evidence for reverse intersystem crossing (RISC) in organic light-emitting devices with small energy gaps between the singlet and triplet excited states is given using the magnetic field effect (MFE) on the electroluminescence (EL) of the devices. The EL decreases when an external magnetic field is applied, indicating that the RISC process occurs and promotes the exciton in the device. The MFE could be a good tool for exploring physical processes in organic semiconductors, especially when spin effects are involved.