Emphasis is placed on solid state properties, which may play an important role in colloid phenomena. The magnitudes of zeta potentials and double layer capacities are affected by the presence of a diffuse charge layer in the solid phase. Several possible models of double diffuse double layers are discussed. A calculation is given of the adsorption curves of Ag+ and I– on AgI, assuming diffuse charge layers in the solid as well as in the liquid phase. A good fit with the experimental data could be obtained with two parameters: the fraction of the number of lattice defects in AgI, 0.17 %, and a potential difference between the surface and the interior of the AgI-crystals, –53 mV, both at 20°C.