During one year, the atmospheric activity concentrations of the short-lived “radon” (²²²Rn) and “thoron” (²²⁰Rn) decay products were measured at Jungfraujoch (3450 m a.s.l.). The measurements were performed using a modified epiphaniometer, with a time resolution of 1 h. In addition, also the long-lived radon decay product ²¹⁰Pb was measured with a time resolution of one month. Strong seasonal variations of all activity concentrations were found, the average summer values being a factor of ten higher than the average winter values. During summer time the activity concentrations of the radon and thoron decay products showed. pronounced diurnal variations. During winter these diurnal variations were absent, and the activity concentrations were close to free tropospheric background values. From the ratio between the activity concentrations of the radon decay products ²¹⁴Pb and ²¹⁰Pb a mean residence time of aerosol particles in the atmosphere of about six days could be estimated. The average activity concentrations of the radon and thoron decay products at Jungfraujoch were found to be related to the regional soil temperatures at a near-by low altitude Swiss Midland site and to the static stability of the air mass between this Midland site and the Jungfraujoch. Furthermore, a significant correlation was found between the radon and thoron decay product activity concentrations and synoptic situations with higher than average activity concentrations for air flours from NE to SW and lower than average concentrations for air flows from W to N.