The design, operation and performance of a unit for dispensing standardised aqueous solutions of 222Rn is described. The unit consists of a sealed polyethylene capsule (containing a known weight of a 226Ra standard solution) immersed in water inside a cylindrical stainless-steel accumulation chamber. The chamber is fitted with valves at each end and has a central bellows so that the chamber can be compressed to dispense solution. The degree of compression, and thus the weight of solution dispensed, is controlled by a microprocessor-controlled motor-drive mechanism. The solution is dispensed via a needle positioned beneath the lower valve.