Prior to 2002, no comparisons of radon activity concentration had been performed within the scope of the EUROMET. The comparison presented here fulfils the need to provide confidence in the capability of national metrology institutes and calibration laboratories to measure radon activity concentration in air. This has been achieved by the measurement of radon reference atmospheres using an electronic, radon measuring instrument as the transfer comparison device (CD). Each of the twelve institutes, from nine countries, calibrated the CD using standard calibration procedures according to their quality system, at three different activity concentrations: 1 kBq/m3, 3 kBq/m 3 and 10 kBq/m3. For each activity concentration, a comparison reference value and its uncertainty have been derived. From these the degrees of equivalence have been calculated and are presented graphically and in the form of matrices. All the results are consistent within the assigned uncertainties. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by EUROMET, according to the provisions of the Mutual Recognition Arrangement (MRA).