A quantitative understanding the origin of sulfur isotope mass-independent fractionation (MIF) is essential to a full interpretation of the Archean sulfur geochemical record. Laboratory experiments have demonstrated that a MIF signature is present in elemental sulfur produced during SO2 photolysis, but the underlying mechanism remains unknown. Here, I report the results of atmospheric chemistry modeling of isotope-selective photodissociation of SO2 in the 1B2 − 1A1 bands from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Self-shielding in the rotationally-resolved lines of 32SO2 produces MIF signatures in SO and residual SO2. A self-shielding origin for sulfur MIF implies that the variations observed in Δ33S in Archean rocks reflect variation in atmospheric SO2 concentration, and demonstrates that MIF in terrestrial rocks can be derived from photochemistry independent of molecular symmetry.