High surface area charcoals are used to remove noxious vapor from gas streams. Recently our laboratory has developed a simple and reliable method to determine a filter's residual adsorption capacity under dry and humid conditions. Work has been performed on ASC whetlerite saturated to various extents with an “irreversible” adsorbed adsorbate (dimethyl methylphosphonate). Data are presented which illustrate how the retention time of a 1 ml(NTP) pulse of methane can be used to quantify the fraction of unoccupied adsorption sites for a strong adsorbent when a filter is saturated with water or dry. In addition, data are presented which suggest that with some charcoals the retention time of methane can be used to determine a filter's residual absorption capacity regardless of its previous water loading history.