The influence of electron self-exchange on the magnetic field effect of various exciplex–radical ion pair (RIP) systems (1,2- and 1,4-dicyanobenzene in combination with the fluorophores pyrene, N-methylcarbazole and N-ethylcarbazole in tetrahydrofurane) has been investigated using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By varying the concentration of the diamagnetic counterpart of one of the radicals in the RIP, the dependence of the linewidth on the electron self-exchange rate has been studied. The functional behaviour of the linewidth-related parameter B_1/2 with respect to the molecule concentration is governed by two opposing effects: lifetime uncertainty broadening of spin levels leads to a linear increase at low concentrations, whereas weakened hyperfine coupling of the corresponding radical ion at high concentrations causes B_1/2 to decrease again until a saturation value is reached. From the linear relationship at low concentrations an estimate of the self-exchange rate constant is obtained.