The electrochemical reaction of hydroquinone-benzoquinone (HQ-BQ) system in acetonitrile was studied by a flow-through Electrochemical Surface Plasmon Resonance (EC SPR) technique The semiquinone radical anion (BQ(-)) was detected as a large negative SPR shift for the first time by the hyphenated technique The real time SPR signal shows the entire reaction process of the BQ(-) including generation transformation and disappearance The lifetime of the BQ(-) in the reaction was extracted by comparing the SPR signals along the flow stream The study demonstrates that the flow-through EC-SPR is a powerful tool for monitoring not only surface bindings but also solution phase chemical reactions In addition it is especially suitable for detecting unstable intermediate reaction products which are difficult to measure using conventional analytical methods (C) 2010 Elsevier B V All rights reserved