In ion-annihilation electrochemiluminescence (ECL), luminophore ions are generated by oxidation as well as reduction at electrodes surfaces, and subsequently recombine into an electronically excited state, which emits light. The intensity of the emitted light is often limited by the kinetic rate of recombination of the luminophore ion species. Recombination or annihilation rates are high ranging up to approximately 1010 M−1 s−1 and can be difficult to determine using scanning electrochemical microscopy or high-frequency oscillations of an electrode potential. Here, we propose determining annihilation kinetics by measuring the relative change of the emitted light intensity as a function of luminophore concentration. Using finite element simulations of annihilation ECL in a geometry of two closely spaced electrodes biased at constant potentials, we show that, with increasing concentrations, luminescence intensity crosses over from a quadratic dependence on concentration to a linear regime—depending on the rate of annihilation. Our numerical results are applicable to scanning electrochemical microscopy as well as nanofluidic electrochemical devices to determine fast ion-annihilation kinetics.