Oleums are mixtures of sulfur trioxide in sulfuric acid and are produced in several strengths. Chlorosulfonic acid (CSA) is formed by reacting sulfur trioxide with hydrochloric acid (HCl). These chemicals are used as sulfating and sulfonating agents in many applications. When accidentally released from vessels or pipes, they react instantaneously with water available from. all sources like atmosphere, concrete, soil, and so forth, to form a fine acid mist that disperses downwind, based on atmospheric conditions. Unlike most other chemicals, the vaporization of sulfur trioxide from oleum spills depends not just on its partial pressure, but a variety of conditions. Complex chemical reaction and beat generation that occur in the liquid phase determine the amount of sulfur trioxide released above a pool of liquid. The sulfur trioxide or CSA vapors then react instantaneously., with the moisture in the atmosphere generating sulfuric acid and/or HCl and beat. Water and/or foam are used effectively in mitigating oleum and CSA spills. However; very limited laboratory or field data are currently available that describe the complex behavior of these chemicals. Several theoretical models have been developed to predict the vaporization and dispersion of the chemical upon loss of containment. These models and methods have not been validated. In this article, details are provided on field tests conducted in Nevada in 2003 and 2006 for spills of 65% oleum and CSA. A description of the spill mitigation, field measurement methods, and some preliminary results are included. (c) 2008 American Institute of Chemical Engineers Process.