The electrochemical and chemical behaviour of nitrosobenzene (PhNO) in acetonitrile has been investigated through cyclic voltammetry and potential step experiments in combination with optical absorption measurements in a cell equipped with a Pt-mesh working electrode. In addition, diffusion layer imaging provided spatially resolved information on the occurrence of secondary processes. The absorption band of an intermediate with maximum at 320 nm is tentatively assigned to the primary anion radical PhNO(-). Secondary reactions of this radical have been found to proceed along two irreversible routes ie by nucleophilic attack in the para-position of unreacted PhNO, or by association processes which lead to azoxy- and azobenzene formation. Addition of a proton donor enhances this second route. In the first route a product anion radical is formed which exhibits a strong absorption with maximum at 550 nm. It can reversibly be oxidized to yield a rather unstable dimer. A weak absorption around 390 nm may tentatively be attributed to this product. In the azoxybenzene-azobenzene route of reaction, the absorptions for both intermediate anions with maxima at 460 nm and 420 nm, respectively, were identified. A reaction mechanism is presented to illustrate the reaction pathways discussed in this investigation.