In-cloud chemistry has important ramifications for atmospheric particulate matter formation and gas-phase chemistry. Recent work has shown that, like hydrogen peroxide (H.sub.2 O.sub.2 ), the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH) oxidize sulfur dioxide dissolved in cloud droplets (SO.sub.2,aq) to sulfate. The work revealed that the pathway of SO.sub.2,aq oxidation with ISOPOOH differs from that of H.sub.2 O.sub.2 . We investigate the chemical mechanisms of oxidation of SO.sub.2,aq with ISOPOOH in the cloud-relevant pH range of 3-6 and compare them with the previously reported mechanisms of oxidation of SO.sub.2,aq with H.sub.2 O.sub.2, methyl hydroperoxide and peroxyacetic acid. The organic products of the reaction are identified, and two pathways are proposed. For 1,2-ISOPOOH, a higher yield pathway via proposed radical intermediates yields methyl vinyl ketone (MVK) and formaldehyde, which can react to hydroxymethanesulfonate (HMS) when SO.sub.2,aq is present. A lower yield non-fragmentation oxygen addition pathway is proposed that results in the formation of isoprene-derived diols (ISOPOH). Based on global simulations, this mechanism is not a significant pathway for formation of MVK and formaldehyde relative to their gas-phase formation but, as previously reported, it can be regionally important for sulfate production. The study adds to previous work that highlights similarities and differences between gas-phase and cloud-droplet processing of reactive organic carbon.