A study of the occurrence of ducts valid at GPS frequencies in the European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric fields is presented. A radio occultation simulator has been applied to determine the mean location of the occultations for a 10 day period in May 2001. In total, 5348 occultations were found. Refractivity profiles have been calculated at the mean occultation location by extracting temperature and water vapor profiles from ECMWF data. The gradient of refractivity with respect to altitude was analyzed for ducting conditions, which occurs below about -160 km-1. About 10% of the simulated measurement profiles show ducts. Polar ducting events are mainly occurring over land, very close to the surface. Midlatitude ducting events are almost evenly distributed over land and sea, where land events are close to the surface too. Tropical ducting events are mainly located over the sea. Events over the sea show ducts within a vertical altitude interval of 0--2.5 km. Tropical events tend to occur at higher altitudes than midlatitudes ones do. The thickness of the layer over which ducting conditions occur increases with altitude and reaches about 120 m for higher altitudes. Near-surface ducting events show the lowest refractivity gradients. Higher up in the atmosphere the gradient increases, where tropical events are generally lower. The found gradient does not support ducting conditions above about 2.5 km. On the basis of our study, about 4% of all occultations can be affected by ducts in the altitude range of 0.2--2.5 km. The number of affected occultations depends on the latitude band; none are found at polar latitudes, 2% are at midlatitudes, and about 10% are at tropical locations.