The steady-state electrochemiluminescence (ECL) from the annihilation reaction of the radical anion and cation of 9,10-diphenylanthracene (DPA) was examined at a double-band microelectrode in acetonitrile, propylene carbonate, 1,2-dimethoxyethane, acetonitrile-toluene, and propylene carbonate-toluene mixtures, all containing 0.1 M tetrabutylammonium hexafluorophosphate. In each solvent, the ECL efficiency was found to be greatest at large DPA concentration due to the effect of competing side reactions. The ECL efficiencies extrapolated to infinite DPA concentrations were found to be solution dependent and were correlated to the separation of the half-wave potentials for the generation of the radical anion and cation. In acetonitrile-toluene mixtures, the ECL efficiency was also found to increase as the ionic strength of the solution was decreased. In 50:50 acetonitrile-toluene the ECL efficiency increased from 6.3 to 8.8% as the concentration of supporting electrolyte was lowered from 100 to 1 mM. This increase was found to correlate with changes in the rate constant for dissociation of the reaction encounter complex as the ionic strength of the solution was lowered.