The short-lived intermediate N,N-dimethylaniline (DMA) cation radical, DMA.+, was detected during the oxidation of DMA in MeCN with 0.1 M tetra-n-butylammonium hexafluorophosphate. The detection was accomplished at steady state by scanning electrochemical microscopy (SECM) with ultramicroelectrodes using the tip generation/substrate collection mode. Cyclic voltammetry (CV) with a 2 mm Pt electrode indicates that DMA oxidation in acetonitrile is followed by a dimerization and two electrochemical reactions, which is consistent with previous results. The DMA.+ intermediate is detected by SECM, where the DMA.+ generated at the ca. 500 nm radius Pt tip is collected on a 5 mu m radius Pt substrate when the gap between the tip and the substrate is a few hundred nanometers. Almost all of the DMA.+ is reduced at the substrate when the gap is 200 nm or less, yielding a dimerization rate constant of 2.5 x 10(8) M-1 s(-1) based on a simulation. This is roughly 3 orders of magnitude larger than the value estimated by fast-scan CV. We attribute this discrepancy to the effects of double-layer capacitance charging and adsorbed species in the high scan rate CV.