Measurements are presented of the electrical double layer and van der Waals forces between the (0001) surfaces of two single-crystal sapphire platelets immersed in an aqueous solution of NaCl at pH values from 6.7 to 11. The results fit the standard Deryaguin-Landau-Verwey-Overbeek (DLVO) theory, with a Hamaker constant of 6.7 X 10-20J. These are the first measurements made using the Israelachvili surface forces apparatus without mica as a substrate material, and they demonstrate the possibility of using this technique to explore the surface chemistry of a wider range of materials. © 1988, Materials Research Society. All rights reserved.