Supersaturation necessary for condensation of a vapor upon ions is maintained continuously by the diffusion of an initially warm saturated vapor through a noncondensing gas into a refrigerated region. Convection is avoided by diffusion vertically downward between a horizontal heated roof and refrigerated floor. Calculations show what flux of vapor and what roof and floor temperatures are necessary. Experiment shows that this diffusion cloud chamber operates as expected, and with reasonable precautions it is quite stable against turbulence. Successful operation depends on avoiding production of condensation nuclei which will make a diffuse rain of condensation in the chamber. Vapor from liquid in a glass flask heated by radiation from above has been found to be practically free of aggregates. The chamber cannot produce satisfactory ion tracks in the presence of too great an average ion load. In the steady‐state operation of the present apparatus the normal background ionization without shielding loads the chamber so close to its limit that most tracks are diffuse. When first applying refrigeration there is a transient condition in which many more tracks are well defined. There are several possible ways of improving the steady‐state ion load capacity of the apparatus.