Reactions of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) with silyl enol ethers, silyl ketene acetals, allylsilanes, enamino esters, and diazomethanes have been studied in CH3CN and CH2Cl2 solutions. The second-order rate constants for C attack at DDQ (log k(C)) correlate linearly with the nucleophile-specific parameters N and s(N) and are 2-5 orders of magnitude larger than expected for SET processes, which strongly supports the polar mechanism for C-C bond formation. The second-order rate constants for O attack agree well with the calculated rate constants for rate-determining single electron transfer (SET). As a radical clock experiment ruled out outer sphere electron transfer, an inner sphere electron transfer mechanism is suggested for O attack.