We report experimental measurements of the contact angle for four liquids on four different silica substrates, the systems covering a wide range of wettabilities. One of the substrates is a smooth planar silica surface, while the others have rough surfaces and meso-pores. We discuss the measured contact angles in relations to the microscopic wetting parameter, α_w. This parameter emerges naturally from a corresponding states analysis of the partition function for this system, and is a measure of the ratio of the liquid–substrate intermolecular interaction to the interaction between two of the liquid molecules. Thus, it is a well-defined measure of wettability at both the nano- and macro-scales. The microscopic wetting parameter is shown to be a monotonic function of the contact angle. The contact angles for the materials with rough surfaces are found to be larger than those for the smooth planar surface for all liquids studied, including both non-wetting and wetting liquids. These results are discussed within the framework of a modified Cassie–Baxter model, in which only a fraction f of the liquid–solid interface is in actual contact with the solid. This fraction f is shown to increase as the wetting parameter increases in a physically reasonable way.