Chlorine oxides play a crucial role in ozone depletion, with the final oxidation steps leading to the formation of chloric (HClO3) and perchloric (HClO4) acids. These acids are widely detected in the atmosphere, especially in the Arctic, and their formation has been primarily attributed to gas-phase reactions. A recent computational study suggested that uptake of gas-phase ClClO3 at the air–water interface can catalyze its hydrolysis to HClO3 or chlorate (ClO3-), highlighting the potential importance of aerosol surfaces. However, little is known about the possible direct formation of chlorine oxides on aerosol surfaces, despite the abundance of chlorine-containing species in aerosols and growing evidence of the high chemical reactivity of these surfaces. In this study, we exposed sea salt crystals to water vapor at relative humidities (RH) ranging from 7.5% to 18.2% and continuously monitored their surface chemical composition. The experiments were conducted using Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) at the MAX IV synchrotron in Lund, Sweden. The crystals were formed by evaporation of Arctic Ocean seawater collected during the Atmospheric River and Onset of Sea Ice Melt (ARTofMELT) campaign onboard the Swedish icebreaker Oden between May and June 2023. These samples are highly relevant to the Arctic atmosphere, as sea salt particles emitted from seawater represent a major aerosol type in the region. The results reveal the formation of highly oxygenated chlorine oxides, most likely HClO3 or ClO3-, upon exposure to water vapor. These findings demonstrate a previously unrecognized pathway for chlorine oxide formation on sea salt aerosol surfaces, with important implications for atmospheric chlorine cycling and ozone depletion processes. Nevertheless, the precise identification of the observed species and the underlying chemical mechanisms remain unclear. Computational work will be essential to resolve these open questions and clarify the reaction pathways involved. Such work will provide insight into our experimental observations and their broader atmospheric significance.