AbstractA continuous flow vapour diffusion chamber has been designed and constructed so that ice nuclei can be activated over ranges of temperature and supersaturation similar to those found in natural clouds. A conventional filter‐processing chamber was also built for carrying out comparative measurements. Simultaneous use of the two chambers under similar conditions of temperature and supersaturation has shown that the filter method leads to an underestimate of the concentration of ice nuclei in an air sample. The ratio of the two simultaneous measurements is strongly dependent on the concentration of Aitken nuclei in the air sample and is also dependent on the chamber supersaturation. Some tentative evidence of the presence of condensation‐freezing nuclei active above water saturation is presen