The report, discussing sulfur trioxide (SO3), is one of a series addressing the prevention of accidental releases of toxic chemicals. SO3, a clear oily liquid or solid at typical ambient conditions, has an Immediately Dangerous to Life and Health (IDLH) concentration of 20 ppm, which makes it an acutely toxic hazard. Reducing the risk associated with an accidental release of SO3 involves identifying some of the potential causes of accidental releases that apply to process facilities than manufacture or use the chemical. The manual identifies examples of potential causes and measures that may be taken to reduce the accidental release risk. Such measures include recommendations on plant design practices, prevention, protection, and mitigation technologies, and operation and maintenance practices. Conceptual cost estimates of example prevention, protection, and mitigation measures are provided.