Nucleation site density measurements have been made for water on brass and stainless steel surfaces using a gas nucleation technique. It has been observed that nucleation sites are substantially more populated on the brass surface compared with the stainless steel surface. The difference is attributed to variations in the detailed surface structure. A vertical scanning interferometer has been used to gather statistical data on cavity mouth diameters and cavity half cone angles for the brass and stainless steel surfaces. The statistical data have been used to predict the nucleation site density, and satisfactory agreement is fortuitously obtained for the brass surface, while poor agreement is obtained for the stainless steel surface. Due to the large uncertainty in the probability of finding cavities that satisfy the gas trapping criteria, the statistical method does not appear to be useful for predicting nucleation site density on surfaces with randomly distributed cavities. It has been observed that only deep cavities are suitable for trapping vapor. (C) 2004 Elsevier Ltd. All rights reserved.