The oxidation of SO<sub>2</sub> to sulfate on sea salt aerosols in the marine environment is highly important because of its effect on the size distribution of sulfate and the potential for new particle nucleation from H<sub>2</sub>SO<sub>4</sub> (g). However, models of the sulfur cycle are not currently able to account for the complex relationship between particle size, alkalinity, oxidation pathway and rate – which is critical as SO<sub>2</sub> oxidation by O<sub>3</sub> and Cl catalysis are limited by aerosol alkalinity, whereas oxidation by hypohalous acids and transition metal ions can continue at low pH once alkalinity is titrated. We have measured <sup>34</sup>S/<sup>32</sup>S fractionation factors for SO<sub>2</sub> oxidation in sea salt, pure water and NaOCl aerosol, as well as the pH dependency of fractionation. <br><br> Oxidation of SO<sub>2</sub> by NaOCl aerosol was extremely efficient, with a reactive uptake coefficient of ≈0.5, and produced sulfate that was enriched in <sup>32</sup>S with α<sub>OCl</sub> = 0.9882±0.0036 at 19 °C. Oxidation on sea salt aerosol was much less efficient than on NaOCl aerosol, suggesting alkalinity was already exhausted on the short timescale of the experiments. Measurements at pH = 2.1 and 7.2 were used to calculate fractionation factors for each step from SO<sub>2</sub>(g) → multiple steps → SO<sub>OCl</sub><sup>2−</sup>. Oxidation on sea salt aerosol resulted in a lower fractionation factor than expected for oxidation of SO<sub>3</sub><sup>2−</sup> by O<sub>3</sub> (α<sub>seasalt</sub> = 1.0124±0.0017 at 19 °C). Comparison of the lower fractionation during oxidation on sea salt aerosol to the fractionation factor for high pH oxidation shows HOCl contributed 29% of S(IV) oxidation on sea salt in the short experimental timescale, highlighting the potential importance of hypohalous acids in the marine environment. <br><br> The sulfur isotope fractionation factors measured in this study allow differentiation between the alkalinity-limited pathways – oxidation by O<sub>3</sub> and by Cl catalysis (α<sub>34</sub> = 1.0163±0.0018 at 19 °C in pure water or 1.0199±0.0024 at pH = 7.2) – which favour the heavy isotope, and the alkalinity non-limited pathways – oxidation by transition metal catalysis (α<sub>34</sub> = 0.9905±0.0031 at 19 °C, Harris et al., 2012a) and by hypohalites (α<sub>34</sub> = 0.9882±0.0036 at 19 °C) – which favour the light isotope. In combination with field measurements of the oxygen and sulfur isotopic composition of SO<sub>2</sub> and sulfate, the fractionation factors presented in this paper may be capable of constraining the relative importance of different oxidation pathways in the marine boundary layer.