This theoretical work examines some basic and overlooked concepts that sustain the efficiency of electrochemiluminescence (ECL) generated by co-reactant systems such as the, nowadays classical, alkyl amine/transition metal(II) complex systems. Changes in the ECL intensities emitted by these systems are much more dependent on the relative diffusivities of the two co-reactants than on the range of thermodynamic and kinetic rate constants that are possible to explore and vary. In particular, decreasing the diffusion coefficients of the metal complexes species (e.g. by adequate redox or photochemically inert large substituents) relative to that of the amine co-reactant leads to a great enhancement in the ECL intensity of the first ECL wave, namely, that observed at the level of the amine oxidation peak. Though investigated by using simulations based on the thermodynamic and kinetic constants of the most common tri-n-propylamine/Ru(bpy)32+ system (bpy=2,2′-bipyridine), the conclusions of this work are more general.