The photochemistry of chlorine dioxide (OClO) in aqueous solution was investigated by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 400 nm, the transient absorption dynamics were probed in the spectral range from 400 to 220 nm. As expected from earlier studies, the main photolytic products ClO + O, formed with a quantum yield of similar to 90%, disappear through fast geminate recombination producing OClO in the electronic ground state. The total quantum yield for chlorine atom production is (PhiP(Cl)) similar to 10%, with the chlorine atom production occurring through two competing processes. The dominant channel for chlorine atom production involves the formation of a short-lived intermediate on a similar to6 ps time scale with a quantum yield of 8 +/- 2%. The remaining 2 +/- 1% is formed through the formation and decomposition of ClOO. The lifetime of ClOO was found to be similar to0.32 ns, in very good agreement with the result of a recent time-resolved resonance Raman study. Finally, the UV absorption spectrum for aqueous ClOO is reported and compared with previously reported spectra obtained in condensed media.