The Gibbsian Surface Excesses (GSE) are the true experimental variables for measurement of equilibria and kinetics for pure and multicomponent gas-solid adsorption. The gravimetric and volumetric methods, which are the most commonly used experimental procedures, are revisited. The common practice of ignoring helium adsorption during measurement of adsorbent density by helium pycnometry and measurement of void volumes in adsorption apparatus by helium expansion can lead to large errors in estimating GSE at high pressures and for less selectivity adsorbed components of a gas mixture. Methods to overcome these problems are described.