The electroreduction of p-benzoquinone (BQ) in unbuffered aqueous solution was studied by electrochemical and spectroelectrochemical techniques. p-Benzoquinone anion radical formed in the process of p-benzoquinone electroreduction in aqueous solution was confirmed by in situ electron spin resonance spectrum for the first time. In situ transmission UV-visible spectra of electrogenerated BQ anion radical were also studied in channel cell. In aqueous solution BQ anion radical was not generated from the one-electron direct electroreduction of BQ, but from the comproportionation reaction of benzoquinone dianion and the parent benzoquinone. A mechanism of radical formation and decay is proposed, and the rate constant of BQ anion radical decay is estimated according to the approximation simulation solution of the postulated mechanism in a channel cell.