Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Kohler theory relating the critical super-saturation S//c to the dry size. Aerosols produced by condensing NaCl showed the same S//c as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold. The investigation also gives insights into the performance of the equipment used, especially the commerical electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in the authors laboratory.