High level electronic structure calculations show that chlorine nitrate is readily hydrolysed in aqueous sulfate aerosols. For the reaction catalysed by H2SO4 and three water molecules, the reaction products involve H3O+NO3-/H2SO4/HOCl. Catalysis by HSO4- leads to an essentially spontaneous reaction, where the reaction products involve NO3-/H2SO4/HOCl or HONO2/HSO4-/HOCl depending on the size of the water cluster.