In this paper we address problems associated with the calculation of the surface excess as traditionally carried out for gas adsorption on surfaces. This calculation is done through a series of steps, one of which is the application of the BET equation for the determination of surface area. The BET theory, despite its popularity, has many unjustified assumptions, requires a choice of the molecular projection area and arbitrary adjustment of the relative pressure range of the BET plot to suit the requirement of a linear best fit. The surface excess, in terms of mol per unit surface area, is therefore subject to compounded errors resulting from these factors. We quantify this with a detailed computer appraisal of adsorption of argon on a graphite surface under sub-critical and supercritical conditions. Careful consideration is paid to the bulk gas volume in the adsorption cell, and it is found that an incorrect estimate of the void volume can lead to a significant error in the surface excess per unit area. This is more pronounced with supercritical adsorption because for a given mass of adsorbent even a 0.1% overestimation of the void volume can change a correct positive surface excess to a negative one at high pressures. (C) 2010 Elsevier Ltd. All rights reserved.