The effect of a magnetic field on the current and power conversion efficiency of protiated and fully (97%) deuterated aluminium tris-8(hydroxyquinoline) (AlQ3) organic light emitting diodes, both with and without a hole transport layer, has been studied and fitted using the triplet–polaron interaction model. Three processes with different magnetic saturation fields were found to be occurring; intersystem crossing between polaron pair states, intersystem crossing between exciton states, and triplet–polaron interactions. The differences in magnitude of the processes are responsible for the overall lineshapes of the magnetic field effects on current and efficiency. Comparison of the magnetic field effects on the protiated and deuterated devices revealed that altering the isotopic content of the material only affects intersystem crossing between the polaron pair states, and the inclusion of a hole transport layer has no effect on any of the processes. Therefore, it appears that there are several underlying spin mixing mechanisms that are responsible for organic magnetoresistance.