We have examined the photochemical formation of hydroxyl radical (•OH) in aqueous extracts of supermicron sea-salt particulate matter (SS PM) collected from the coast of northern California. All extracts formed •OH during illumination, indicating that this process is widespread in sea-salt particles. Scaling extract results to SS PM conditions reveals that in situ rates of •OH photoformation are typically 100–1000 μM hr−1 in midlatitude sea-salt particles (summer, midday, 88% relative humidity). These rates are comparable to calculated rates of partitioning of gas phase •OH to the particles and are 3–4 orders of magnitude greater than •OH photoformation rates in surface seawater. Photolysis of nitrate was a dominant source of •OH in the particle extracts, accounting for an average of 59 ± 25% of its formation. The other sources of •OH have not been identified, but photolysis of organic compounds derived from seawater is likely important. The lifetimes of •OH in the sea-salt particles are of the order of 10−9–10−8 s and are primarily controlled by reactions with unidentified, but probably organic, compounds. Bromide and chloride are also significant sinks of •OH, typically accounting for approximately 25% of its loss. The rapid formation and destruction of •OH in sea-salt particles likely significantly affects the chemistry of halides, organic compounds, and other reduced species in SS PM. In turn, these particle reactions probably alter the budgets of gases such as ozone and volatile organic compounds in the marine boundary layer.