Abstract: Radical pair reactions can explain phenomena ranging from avian magnetoreception to spin-dependent charge-carrier recombination and transport rates in semiconductor materials. Central to the radical pair model are weakly-coupled electron spin pairs in a matrix with weak spin–orbit interactions. However, while it has been known that the magnetic-dipolar and spin-exchange interaction strengths within these pairs are weak, specific values within their native operating environment (e.g. within organic light emitting diodes) have been experimentally difficult to obtain. To probe intra-pair coupling strengths in situ for an organic semiconductor diode under operating conditions, electrically detected magnetic resonance is used to measure the detuning behavior of spin-Rabi nutation frequencies. In the limit of negligible exchange and dipolar coupling, a fundamental Rabi frequency is analytically predicted to be accompanied by a first harmonic at twice the fundamental frequency. Deviations from this analytical prediction are due to finite values of exchange and dipolar interactions, allowing constraints on both the exchange and dipolar coupling energies to be formulated.