A total of 35 rapid small-scale column tests (RSSCTs) were performed to investigate the effects that influent total organic carbon (TOC) concentration, empty-bed contact time (EBCT), field-scale granular activated carbon (GAC) particle size, and small-scale GAC particle size have on TOC breakthrough in a GAC adsorber. Results from the extensive experimental matrix of RSSCTs systematically demonstrate the following: increasing the influent TOC concentration decreased the time for TOC to breakthrough a GAC adsorber; increasing the adsorber EBCT led to non-proportional increases in TOC breakthrough times and apparent efficiencies of scale; decreasing the field-scale GAC particle size from 8x20- to 12x40-mesh nearly doubled the TOC breakthrough time for all EBCTs and for all fieldscale GAC particle sizes tested; and, decreasing the small-scale GAC particle size from 60x100- to 100x200-mesh non-proportionally decreased the TOC breakthrough time, but the effects were only observed for the tests performed with the large 8x20-mesh field-scale GAC at relatively small EBCTs (5- and 10-minutes). Includes 6 references, tables, figures.