Optically transparent ultramicroelectrodes (OT-UMEs) comprising carbon nanotube (CNT) networks on quartz, are introduced and used to monitor quantitatively the interfacial concentration of tris(2,2'-bipyridine)ruthenium(II) during cyclic voltammetry. The OT-CNT-UMEs combine the attractive properties of small-scale electrodes, e.g., high diffusion rates and good signal-to-noise, with the ability to probe electrochemical processes optically, from the rear of the electrode. This enables optical measurements of the solution, close to the electrode surface, without significant interference from absorption or scattering processes.