This paper summarizes the key characteristics of the United States Environmental Protection Agency's (U.S. EPA's) Mercury Measurement Toolkit and the experiences using it to develop emission factors for coal-fired power plants in China, Russia, and South Africa. The U.S. EPA developed and tested a portable mercury sampling and analytical laboratory (the U.S. EPA's Mercury Measurement Toolkit). The U.S. EPA's Mercury Measurement Toolkit is designed to efficiently and economically determine speciated mercury emissions on site at coal-fired facilities. The U.S. EPA, working with the United Nations Environment Programme (UNEP) and in-country experts, used the U.S. EPA's Mercury Measurement Toolkit to develop accurate, timely, reliable, and economical mercury measurements. The measurements, when paired with facility operating information (e.g., fuel usage and heat content), were used to calculate mercury emission rates and mass emissions. Additional analyses of solids (e.g., coal and fly ash) yielded estimates of pollution control equipment efficiencies. The U.S. EPA's Mercury Measurement Toolkit may greatly enhance comprehensive emission inventory development by reducing the time and cost of conducting unit level mercury emissions testing.