The influence of electrical double layers on the macroscopic solid-liquid-vapour contact angle φ is investigated. It is shown that the presence of an electrical double layer at the solid-liquid phase boundary accounts for the maximum hydrophobicity of solids at their point of zero charge. Using a simple double layer model, theoretical dependences of φ on the activity of the potential determining ions (pX) and the indifferent electrolyte concentration have been computed. Experimentally observed trends in φ on silver iodide and surface modified quartz agree with the theoretical predictions. A way in which measured φ(pX) relationships shed light on deviations from Nernstian surface potential (pX) behaviour is indicated.