The almost irreversible oxidation of N,N‐diethyl‐p‐phenylenediamine by 12‐tungstocobaltate(III) was studied by means of a multi‐mixing, stopped‐flow apparatus with optical detection. Both unprotonated and simple protonated p‐phenylenediamine are oxidized rapidly to form p‐semiquinonediimine. The reaction rate is dependent on the concentration of all ions present in the solution, as well as on the choice and concentration of the buffer. The rate constant varies from 3·104 M–1 s–1 to 1.4·106 M–1 s–1 at 25°C. The activation energy in all cases is very small.