We have extensively studied the enhancement of the second reduction peak current I-p(c2) of a stepwise one-electron redox reaction at a hanging mercury drop electrode (HMDE) in aprotic solution on the basis of cyclic voltammetric and in situ CCD video measurements. The cyclic voltammograms of the redox reactions of molecular oxygen, 1,4-dinitrobenzene, benzoquinone, 2,3,5,6-tetramethyl benzoquinone, azobenzene, methylviologen dichloride and 9,10-diphenylanthracene (DPA) at HMDE and glassy carbon (GC) electrode in dimethyl sulfoxide solutions containing 0.1 M tetraethylammonium perchlorate (TEAP) were compared. Generally, the I-p(c2) was found to be greater than the first reduction peak current I-p(cl)(i.e., I-p(c2)/I-p(c1) > 1) at HMDE, while the opposite result (i.e., I-p(c2)/I-p(c1) < 1) was obtained at GC electrode. Changing the state (and shape) of the mercury electrode from the liquid (and spherical) (i.e., HMDE) to semi-solid (and planar) (e.g., mercury film-coated gold electrode), increasing the concentration of TEAP and the use of poly(vinylchloride) as a surfactant were found to effectively allow the value of I-p(c2)/I-p(c1) to be closer to 1, indicating that the so-called streaming phenomenon of HMDE results in I-p(c2)/I-p(c1) > 1. From the CCD video of HMDE captured during the electrochromic redox reaction of DPA, it was clarified that the upward streaming of the HMDE surface causes a continuous supply of the redox species from the bulk of the solution to the electrode surface, resulting in the enhanced I-p(c2). (c) 2008 Elsevier B.V. All rights reserved.