Orimulsion is an emulsified fuel, composed approximately 70% Venezuelan bitumen, 30% water, and trace amounts of surfactant, and is being marketed primarily as a base-load fuel for utility boilers. The U.S. Congress requested that the Environmental Protection Agency conduct a study to provide technical information regarding Orimulsion and its potential environmental impacts. The study by EPA team and led by the Office of Research and Development's National Risk Management Research Laboratory (NRMRL) and literature reviews, visit to sites now using Orimulsion, and a series of combustion tests measured mass emissions of carbon monoxide, nitrogen oxides and sulfur, particulate matter, trace metals and organic compounds generated by the combustion of Orimulsion and heavy fuel oil. Potential air-pollution-related issues associated with Orimulsion combustion include elevated levels of sulfur, nickel, and vanadium, and generation of submicron particles and sulfur trioxide. These issues are similar to those associated with heavy fuel oil combustion, and can be addressed by use of appropriately designed and operated pollution control equipment.