Radon 222 (Rn) vertical soundings were made on five different days over Illinois from 150 meters to as high as 5 km above the surface. The Rn concentration ranged from 60 to 300 pc m−3 at 150 meters above the surface and decreased exponentially by approximately one‐half with every 700‐meter increase in elevation. The Rn soundings were compared with earlier theoretical Rn profiles and with actual Rn measurements taken by several other investigators. The Rn concentration was found to decrease more rapidly with increasing altitude than had been calculated theoretically. Possible explanations for this disagreement are presented.