A number of consumer and commercial products may react, upon inappropriate mixing, to produce substances of greater toxicity than the starting materials. Consumer product mixing incompatibilities have been well documented and warning labels appear on high-risk products. In industry, a wider array of potential mixing incompatibilities exists and includes potential accidents in chemical storage and hazardous material handling. Emergency response personnel are a group who often deal with inadvertent mixing related to transportation and other hazardous materials incidents. This article assembles some better-known examples of toxicologically significant exposures resulting from inadvertent or deliberate mixing of incompatible materials.