Previous investigators noted that a mixture of dimorphic Sb 2O 3 oxidizes to orthorhombic Sb 2O 4 more rapidly than each dimorph separately. It was hypothesized that Sb 2O 3(c) sublimes to low-energy nucleation sites on Sb 2O 3(o) to oxidize. In this study thermodynamic calculations indicated that the driving force for oxidizing a sublimed species is much greater than that for nonsublimed Sb 2O 3. The hypothesis was validated experimentally using small-particle-size (10-20 μm) Sb 2O 3 to achieve appreciable sublimation rates. It was found that the oxidation rates of both Sb 2O 3(c) and Sb 2O 3(o) are linearly correlated to the equilibrium sublimation pressure of each dimorph. Also, the apparent activation energy of oxidation was determined to be 42.6-45.9 kcal/mole, approximating the enthalpy of sublimation for Sb 2O 3 (∼43-47 kcal/mole). These observations strongly support the hypothesis that Sb 2O 3 oxidation is sublimation controlled.