The oxidation of N,N-dimethyl-p-phenylenediamine ((R) over bar) by peroxodisulfate (S2O82-) in aqueous solution was studied spectrophotometrically. A complex mechanism which couples a radical chain reaction with an autocatalytic reaction as well as with several consecutive and parallel reactions determines this apparent simple two-step oxidation. Following the formation of only a small amount of N,N-dimethyl-p-semiquinonediimine (S+), peroxodisulfate almost exclusively oxidizes this organic radical to the corresponding quinonediimine (T) over bar), whereas the sulfate radical (SO4-) which is formned by the one-electron reduction of S2O82-preferably oxidizes The rate of this radical chain reaction increases autocatalytically because of a fast synproportionation between and (R) over bar, which regenerates up to two molecules S+ for each molecule S' oxidized by S2O82-(and SO4-). The contribution of this synproportionation on the oxidation of R may become larger than the contribution of the radical chain reaction. During the autocatalytic increase of S+, two consecutive reactions ((R) over bar --> S+ --> (T) over bar) and (S2O82- --> SO4- --> SO42-) are coupled with four parallel reactions ((R) over bar + S2O82- --> S+), (S++ S2O82---> (T) over bar), ((R) over bar + SO4- --> S+), and (S+ + SO4---> (R) over bar). The synproportionation can be regarded as the pump of the overall reaction, Rate determining is the oxidation of S+ by peroxodisulfate. Despite this complex mechanism the rate constants for the parallel oxidation of both (R) over bar and S+ by S2O82-, (k) over bar (1) and k(2), could be determined by a procedure based on a plot of the formation rate of S', measured as change in absorbance, against the corresponding absorbance. At 25 degreesC, pH = 5.4, and ionic strength 0.025 M, the pH-dependent rate constant (k) over bar (1), has the value 1.8 (+/-0.2) M-1 s(-1), whereas the pH-independent rate constant k(2) equals 95 (+/-4) M-1 s(-1).