The electrogenerated chemiluminescence (ECL) process of the Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridyl/co-reactant system at an as-deposited diamond electrode was investigated using cyclic voltammetry (CV) and electrochemical impedance measurements. At an as-deposited diamond electrode, three ECL waves (1.2 V, 2.0 V, 2.3 V vs Ag/AgCl) for tripropylamine (TPrA) were obtained. The anodic potential limit for ECL emission at as-deposited diamond was 2.5 V vs Ag/AgCl due to its wide potential window in the aqueous solution. The potential range of ECL at diamond was much wider than those for glassy carbon (GC) and polycrystalline Pt electrodes, suggesting the possibility of the ECL for amines with higher oxidation potentials. For the ECL peaks at 1.2 V corresponding to the oxidation potential for Ru(bpY)(3)(2+), the current densities and light intensities were increased linearly with the square root of the TPrA concentration. This suggests that TPrA oxidation necessary for the ECL occurred thorough homogeneous electron transfer between Ru(bpy)(3)(3+) and TPrA species. However, for the ECL peaks at 1.9 V, the current densities and light intensities were increased linearly with an increase in TPrA concentration, suggesting that TPrA oxidation was due to direct oxidation at the electrode surface. At the direct TPrA oxidation potential, dealkylation was found to occur and produce the corresponding secondary amine, primary amine, and the appropriate aldehyde. The ECL peak for TPrA at 2.3 V was found to be the ECL for propylamine, generated by the dealkylation. By use of a diamond electrode, even for primary amines, i.e., methylamine (MeA) or ethylamine (EtA) with very high oxidation potentials, the direct oxidation potential could be detected through the ECL process. The behavior of the ECL of TPrA for the as-deposited diamond and GC electrodes was investigated by long-term (1000) potential cycling. For the as-deposited diamond electrode, the light intensity showed stable behavior during the entire cycle. The decrease of the light intensity was only ca. 0.1 of the initial intensity. The high stability for the as-deposited diamond was found to be attributable to the low adsorption property for reaction products on the diamond surface. We could show that the diamond electrode is the promising candidate for the electrode material for ECL with a wide ECL potential region and high stability.