The International Atomic Energy Agency (IAEA) in Vienna and the European Union (EU) in Bruxelles formed the “International Radon Metrology Programme” (IRMP, scientific secretary: F. Steinhausler, University of Salzburg, Austria). The IRMP is designed to assess and foster the improvement of radon and decay product measurements that are made around the world. Within the framework of the IRMP, the U.S. Environmental Protection Agency Radiation and Indoor Environments National Laboratory (EPA) in Las Vegas, Nevada, organized jointly with the U.S. Bureau of Mines an international intercomparison exercise at a former uranium mine (Twilight Mine, Colorado) and the EPA Radon Laboratory. The main objective of this exercise was to compare radon and radon decay product instruments under both wellcontrolled as well as widely fluctuating exposure conditions. The laboratory exposures occurred under relatively steady radon and decay product conditions, with a moderate equilibrium ratio, while the conditions in the mine fluctuated greatly and the equilibrium ratio was low. An additional purpose of the exercise was to provide a forum for manufacturers and measurement organizations worldwide to exchange information and plan improvements in their operations and calibration programs. Altogether 19 organizations from seven countries intercomparing 32 different radon and radon decay product instruments participated in this exercise. This paper summarizes the results from the analysis of the experimental data obtained in the Bureau of Mines Twilight Mine in July of 1994, as well as the results from the EPA Radon laboratory in August of 1994.