Tetrachlorohydroquinone dehalogenase catalyzes two successive reductive dehalogenation reactions in the pathway for degradation of pentachlorophenol in the soil bacterium Sphingobium chlorophenolicum. We have used pre-steady-state kinetic methods to probe both the mechanism and the rates of elementary steps in the initial stages of the reductive dehalogenation reaction. Binding of trichlorohydroquinone (TriCHQ) to the active site is followed by rapid deprotonation to form TriCHQ-2 and subsequent formation of 3,5,6-trichloro-4-hydroxycyclohexa-2,4-dienone (TriCHQ*). Further conversion of TriCHQ* to 2,6-dichlorohydroquinone (DCHQ) proceeds only in the presence of glutathione. Conversion of TriCHQ to DCHQ during the first turnover is quite rapid, occurring at about 25 s-1 when the enzyme is saturated with TriCHQ and glutathione. The rate of subsequent turnovers is limited by the rate of the thiol-disulfide exchange reaction required to regenerate the free enzyme after turnover, a reaction that is intrinsically less difficult, but is hampered by premature binding of the aromatic substrate to the active site before the catalytic cycle is completed.