The strong electroluminescence (EL)-detected magnetic resonance of PPV-based light emitting diodes is compared to the conductivity (σ)- and photoluminescence (PL)-detected resonances. It provides direct evidence that polaron-to-singlet exciton conversion is responsible for the EL. In contrast to the narrow PL-enhancing resonance assigned to polaron recombination, strong EL- and σ-quenching resonances are attributed to the spin-dependent polaron-to-bipolaron decay. The half-field EL- and σ-detected resonances of two distinct triplet excitons is believed to result from triplet-triplet fusion to singlets. © 1992 The American Physical Society.