This is a thorough theoretical study of the photodissociation of ClOOCl. We present ab initio calculations of the potential energy curves and a modification of the semiempirical MNDO-d method, designed so as to reproduce the ab initio results as well as available experimental data. Simulations of the nonadiabatic photodissociation dynamics have been run with a direct semiclassical method, based on the semiempirical wavefunctions and potential energy surfaces. We have run three groups of trajectories, with randomly chosen initial conditions, such as to simulate excitation in three different regions of the absorption spectrum, around 460, 325 and 264 nm, respectively. We find that dissociation to 2Cl + O₂ is the main photoreaction, and a small amount of ClO is formed at the highest excitation energies. The mechanism is mainly sequential at low energies, involving the short lived species ClOO, while at high energies the synchronous Cl–O bond breaking prevails. We compare the computed quantum yields, final fragment energies and anisotropy parameters with the corresponding experimental quantities, measured by Moore et al. (T. A. Moore, M. Okomura, J. W. Seale and T. K. Minton, J. Phys. Chem., 1999, 103, 1691. Ref. 1.) and we propose a partially new interpretation of their observations.