Photofragmentation of chlorine dioxide (OClO) and its aggregates is investigated in the UV regime [349-373 nm, (A) over tilde((2)A(2)) <-- (X) over tilde(B-2(1)) transition]. The products of UV photolysis are subsequently ionized with vacuum UV laser radiation followed by time-of-flight mass analysis. The isolated molecule decays predominantly by formation of vibrationally excited ClO (X((II)-I-2)) as a result of predissociation. Highly vibrationally excited ClO is formed if even quanta of the asymmetric stretching vibration are excited. The results are discussed in relation to the UV absorption cross section and competitive fragmentation routes, such as formation of molecular oxygen. The UV photolysis of homogeneous chlorine dioxide aggregates [(OClO)(n)] yields fragments such as excited molecular oxygen, Cl2O3, and Cl3O5. Evidence for evaporation of neutral molecules from excited aggregates is found as well. The results on aggregate photolysis are discussed in relation to experiments on chlorine dioxide in the gas and condensed phase. Ab initio calculations are performed in order to rationalize the experimental results. Properties, such as structures, stabilities, and vibrational frequencies, of different isomers of the OClO dimer and the photolysis product Cl2O3 are derived. Possible implications to stratospheric photochemistry are briefly discussed.