It is shown that the equilibrium wetting tension, defined thermodynamically as the difference between the solid-vapor and solid-liquid interfacial tensions, can be measured to the same high degree of certainty and accuracy as the surface tension of liquids. Just as for surface tension isotherms the wetting tension isotherms can be analyzed through the Gibbs adsorption equation. This allows for unambiguous evaluation of the adsorption difference at the two interfaces. As an example we have investigated the wetting of smooth and homogeneous silica glass surfaces by solutions of cetyltrimethylammonium bromide (CTAB). At small concentrations (below the point of zero charge) equilibrium monolayer adsorption is larger at the solid-vapor interface than at the solid-liquid interface. At higher concentrations (near the critical micelle concentration) the adsorption difference changes sign due to formation of bilayer at a solid-liquid interface. Adsorption reequilibration within the hydrophobic monolayer occurs when a three-phase contact line is displaced. This reequilibration is slow and explains large contact angle hysteresis typically observed when measurements are done under nonequilibrium conditions.