This research investigates pore volume losses and total organic carbon (TOC) adsorption in virgin and reactivated granular activated carbon (GAC) used during full-scale water treatment. Pore volume changes were studied at several depths within each contactor. Pore volume at upper depths was lost over a wide range of pore sizes, whereas pore volume losses at middle and lower depths occurred primarily in the micropores. Adsorptive forces are inherently strongest in micropores because both pore walls exert an attractive potential on natural organic matter (NOM). Correspondingly, NOM that reached the middle and lower depths would inherently be characterized by a lower affinity for the GAC surface, necessitating double-wall adsorption. Furthermore, the upper depths of the reactivated GACs were characterized by considerable pore volume losses in the 100-500¿ range, where adsorption would inherently represent single-wall adsorption of highly adsorbable molecules. The comprehensive data set has facilitated a mass/volume balance that compared computed volume of NOM removed through the GAC contactors, (monitored via TOC influent and effluent), versus pore volume lost in pores <500¿ width (monitored via argon adsorption tests). These two volumes essentially equaled one another, and this was observed for each of the five loading conditions that were tested herein. Includes 19 references, table, figures.