The growing concern over the radiological consequences of environmental Rn 222 has led to the development of measurement facilities in several European laboratories. A recent EUROMET intercomparison between thirteen laboratories demonstrated an overall spread of results of the order of 40%. It was concluded that this spread might have been due to differences in the measurement techniques, standards and nuclear data used. A second exercise is proposed, similar to the first but preceeded, as necessary, by (i) intercomparisons of standards use for calibration (for example, Ra 226 solutions), and (ii) international agreement on relevant nuclear data. As before, NPL will conduct the exercise by sending a unique set of three glass bulbs, containing measurement quantities of Rn 222 (in nitrogen), to each of the participating laboratories. The bulbs will be fitted with valves, will have a nominal volume of 35 cm3, and will contain approximately 5 kBq of Rn 222 when received by the laboratory. Laboratories measuring the bulbs non-destructively should return them to NPL sealed. If necessary, NPL will provide a participant with an inactive bulb, for calibration purposes.