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 CH₂Cl₂solutions. The second-order rate constants for C attack at DDQ (log k C) correlate linearly with the nucleophile-specific parameters N and sɴ 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.