Soil specific surface area, SA, was determined from hyper-dry-region water retention data obtained by a chilled-mirror dewpoint psychrometer for i) twentyfive soils from the Danish Soil Library representing most Danish soil types, ii) twenty soils from the University of Arizona Soil, Water, and Environmental Science (SWES) Club, representing different locations through the state of Arizona, iii) eight European soils, and iv) six soils from a Danish natural clay gradient. The soil series cover a wide range of clay and organic carbon contents. Pretreatments to remove organic matter were omitted. For the psychrometer data we determined SA at a soil-water potential corresponding to one molecular layer of water coverage. This SA was compared to specific surface areas as determined by the Ethylene Glygol Monoethyl Ether (EGME) method and the nitrogen adsorption (N₂-BET) method. The three different measures of SA was compared and related to basic soil properties. The main discussion will focus on the effects of the clay to carbon ratio on the external and internal surface areas.