ABSTRACT The equilibrium vapor pressures of water are calculated for two different geometric configurations: a liquid cap formed on a single substrate sphere and a liquid pendular ring formed about the contact point of a pair of adhering, identical spheres. The substrate is a structureless, macroscopic (i.e., radius R > 50 nm), relatively hydrophobic sphere. For each configuration, pure water and sulfuric acid solution are used separately as the interface liquid. In addition to the available surface tension measurements of sulfuric acid solution against air, our calculations utilize the tabulated data of activity of water over the sulfuric acid solution and the solution density. The substrate's interfacial tension against air is treated as a parameter in these calculations. Then, by using Young's equation as a constraint in our calculation, we can determine the contact angle of the surface liquid residing on substrate spheres for both configurations. We apply Kelvin's equation in combination with both wat...