The mechanism of the Gibbs reaction, a colorimetric phenol assay that applies N-chlorobenzoquinone imines 1 in an aqueous basic medium, was investigated. It is concluded that N-chloroimine radical anion 7 generated in a single electron transfer (SET) from the anion of phenol 4 to N-chloroimine 1 can produce indophenol dye 3 in three distinct routes. For more reactive reagent-substrate pairs, a route is proposed that involves a fast combination of the radical pair in the solvent cage and, consequently, the total rate of which exhibits a pH-independent second-order kinetics, as does the preceding SET itself. For less reactive reagents, a route is proposed in which the N-chloroimine radical anion 7 escapes from the solvent cage to initiate a chain reaction, evidenced by its characteristic kinetics. It has been found in the kinetic experiments that during propagation the chlorine of the chain carrier N-chloroimine radical anion 7 is substituted by the anion of 4 in a bimolecular rate-determining step. Therefore, the mechanism of the chain reaction is termed S(RN)2. In the case when the anion of 4 is less active, a competitive reaction along a third route can proceed in which the N-haloimine radical anion 7 yields benzoquinone imine 6 by the elimination of halogenide and the abstraction of an H-atom from the medium. Compound 6 is also known to give indophenol 3 with a second-order but pH-dependent rate that is considerably faster than the rate in the first route. On the basis of the different kinetic characteristics outlined above a clear distinction can be made among these three pathways. In this paper, evidence is also presented for the initiating SET. Furthermore, it is of high importance that the N-haloimine radical anion 7 can also be generated from reagent 1 using external electron donors and, independently of its origin, it can be spin trapped with 2,2,6,6-tetramethylpiperidine-N-oxyl.