Meteorological and radon concentration data referring to two measurement campaigns carried out in Milan, Italy, are reported and discussed. An urban mixing height (UMH) is defined through radio sounding data and an equivalent mixing height (EMH) is obtained through application of a simple box model obtained with radon measurements, especially in the meteorological situations found during the campaigns which were characterized by negligible advection. It is shown that the metorological and the radon-model estimations of the mixed layer height are highly correlated. It is concluded that a simple β counter for particulate radon progeny can be regarded as a useful instrument for estimating urban mixing heights in situations dominated by weak thermally induced turbulence which are also responsible for intense photochemical activity and pollution episodes. ¿ American Geophysical Union 1994