For the Borexino solar neutrino experiment a concentration line for 222 Rn from a large volume of nitrogen gas has been constructed. It is based on cryo-adsorption in a charcoal trap of very low intrinsic 226 Ra contamination. Consequently, the blank for activity from the daughter nuclide 222 Rn is very low. 222 Rn is recorded with proportional counters. This allows the detection of Rn at concentrations below the μBq/m 3 level in gaseous nitrogen. The removal of radon from liquid nitrogen is achieved by direct adsorption in the liquid phase. 222 Rn – measurements on evaporated nitrogen with and without previous purification are reported.