Condensation nuclei measurements using a low supersaturation (∼10%) thermal gradient diffusion cloud chamber (TGDCC) and a high supersaturation (∼200%) expansion type instrument were compared on a series of three balloon flights over Laramie, Wyoming. In general the two instruments produced similar vertical profiles but some discrepancies remain unexplained. Agreement between the two would indicate that the low supersaturations used in the TGDCC were still large enough to cause the instrument to count essentially all of the particles present. The TGDCC condensation nuclei (CN) counter was flown at several sites in both the Northern and Southern Hemispheres. The results indicate the existance of a relative maximum in the CN mixing ratio associated with the upper equatorial troposphere and what appears to be a worldwide constant mixing ratio of CN above 20–25 km.