The ultrafast dynamics of chlorine dioxide (OClO) photochemistry in aqueous solution was studied by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 395 nm, the transient absorption dynamics were probed at 12 different wavelengths ranging from 350 to 700 nm, The transient absorption features observed in the visible wavelengths are assigned to correspond to the vibrationally hot photoisomer ClOO*. The spectral dynamics reveal the vibrational relaxation of this molecule in its ground electronic state. The total vibrational energy relaxation occurs within similar to 9 ps. The dynamics of the formation of chlorine atom was examined by measuring the absorption dynamics in the 350-390 nm range. The time constant for Cl formation is calculated to be similar to 200 ps. The data show that the dominant pathway for Cl formation is via the isomerized ClOO molecule. No fast component for Cl is detected, indicating that the ClOO molecule reaches thermal equilibrium before dissociating into Cl and O-2.