The Wilhelmy plate method was used to study the wetting bahvior of CTAB solution on mica at concentrations from 10Ä-7Å M to 3*10Ä-4Å M. A freshly cleaved mica plate shows large amplitude oscillations of the dynamic advancing wetting tension and a jump-stick behavior of the meniscus due to surfactant adsorption at the three-phase contact line. Desorption in pure water is slow. During relaxation measurements, the wetting tension reaches its equilibrium value on a timescale of several hours. The equilibrium wetting tension decreases at low concentrations, reaches a minimum at 2.4*10Ä-5Å M corresponding to a contact angle of 73ÄoÅ, and then increases (solid/liquid bilayer adsorption) up to 3*10Ä-4Å M. The solid/liquid and solid/vapor adsorption isotherms were deduced from application of the Gibbs adsorption equation, surface force, and XPS data. The solid/liquid bilayer contains 1.4-1.6 surfactant cations per mica charge site, 0.7-1.0 in the inner layer and 0.6-0.75 in the outer layer. The areas per surfactant cation are 0.48-0.68 nmÄ2Å and 0.64-0.80 nmÄ2Å.