Condensation of water vapor on various surfaces was studied experimentally. For surfaces with an air-water contact angle theta less than 20 degree , the experimentally determined values of critical supersaturation S//c agreed with those given by the Volmer theory. At higher theta , the experimental values of S//c were below the Volmer theory values. When the applied supersaturation was less than S//c, condensation was avoided for periods as long as 20 h. It was determined both by experiment and theoretical analysis that the effect of surface roughness is to decrease S//c only slightly. These results suggest that most insoluble airborne particles are not likely to serve as cloud condensation nuclei.