The interaction forces between silica surfaces modified to different degrees of hydrophobicity were measured using colloidal probe atomic force microscopy (AFM). A highly hydrophobic silica particle was prepared with octadecyltrichlorosilane (OTS), and the interaction forces were measured against silica substrates modified to produce surfaces of varying hydrophobicity. The interaction forces between the highly hydrophobic particle and a completely hydrophilic silicon wafer surface fitted well to the DLVO theory, indicating that no additional (non-DLVO) forces act between the surfaces. When the silicon wafer surface was treated to produce a contact angle of water on surface of 40, an additional attractive force that is longer ranged than the van der Waals force was observed between the surfaces. The range and magnitude of the attractive force increase with the contact angle of water on the substrate. Beyond the effect on the contact angle, the hydrocarbon chain length and the terminal groups of hydrophobic layer on the substrate only have a minor effect on the magnitude of the force, even when the substrate is terminated with polar carboxyl groups, provided the hydrophobicity of the other surface is high.