We report the electrochemiluminescence (ECL) intensity of tris(2,2 ' -bipyridyl)ruthenium(II) [Ru(bpy)(3)(2+)] as a function of varying solution viscosity in mixtures of N,N-dimethylformamide and glycerin. Diffusion coefficient (D), electron-transfer rate (k) of Ru(bpy)(3)(2+/3+),and luminescence intensity are measured and correlated with viscosity and ECL intensity (I-ecl). I-ecl decreases with increasing solution viscosity and tracks diffusion and electron transfer until it reaches an upper limit. This limit is reached at D = 5 x 10(-7) cm(2)/s and k = 0.03 cm/s in this system. Normalizing I-ecl for diffusion reveals a weak enhancement of ECL with increasing viscosity.