Photodissociation of ClO2 in aqueous solution at 400 nm results in the formation of ClO+O and Cl+O-2. ClO and O geminately recombine to ClO2 in the electronic ground state (B-2(1)), formed with an initial vibrational energy of approximate to 2.5 eV. In this paper the vibrational relaxation of ClO2(B-2(1)) in aqueous solution is studied by femtosecond transient absorption spectroscopy in the spectral range 234 to 1024 nm. The measured transient absorption of the vibrationally relaxing ClO2 molecules is compared with the transient absorption calculated for relaxation in the asymmetric stretch as well as the symmetric stretch and bending modes. The calculations of the absorption spectra pertaining to the asymmetric stretch are based on a harmonic potential derived from the experimentally determined fundamental vibrational energy, whereas that of the symmetrical vibrations are based on ab initio potentials. An excellent agreement is obtained by assuming that the vibrational relaxation predominantly occurs in the asymmetric stretch with a 9.5 ps relaxation time. A weak spectral feature in the ultraviolet part of the spectrum is assigned to vibrational relaxation in the symmetric stretch and bending modes, indicating a coupling between the asymmetric and symmetric modes.