The hyper-arid coastal Namib Desert in southern Africa is characterized by frequent morning fog, which is an important water supply for certain desert organisms. The fog is climatologically driven by both the cool upwelling Benguela Current and the general stability of the lower atmosphere at tropical latitudes. High dimethyl sulfide (DMS) concentrations associated with the productive upwelling zone suggest DMS as the major source of condensation nuclei for fog formation. However, the few available chemical analyses of fog water show a strong relationship between dissolved calcium and sulfur in the fog. A likely source of these ions is the gypsum (CaSO4) crust common across the gravel plains of the Namib. The current study exploits the isotopic difference between sulfur of the gypsum crusts (δ34S = +12‰) and that of marine sulfur (δ34Sseasalt = +20‰, δ34SDMS = +18‰) to determine sources of sulfur in size-segregated aerosols collected before, during, and after a fog event in the Central Namib, 56 km from the coast. Any significant contribution from the relatively infrequent releases of H2S (δ34S = -20‰) from the organic-rich Benguela sediments will also be apparent. A previous study of bulk chemical concentrations in Namib aerosols from the same study area found a biogenic marine signal in the fine fraction (<1 micron), whereas the coarse fraction (>1 micron) had a mixed inorganic marine (sea salt) and terrestrial signature. The implications of the aerosol-fog chemistry relationships are explored, including a possible feedback between aridity and fog formation.