A rapid small-scale column test (RSSCT), designed according to the assumption of no dependency of the intraparticle surface diffusion coefficient on the activated carbon particle size, was able to accurately predict breakthrough of three volatile organic chemicals as well as background organic matter under both oxic and anoxic conditions. A second RSSCT, however,--designed assuming a linear dependency of the intraparticle surface diffusion coefficient on the activated carbon particle size--significantly overestimated the adsorptive capacity attainable from the large-column tests. The presence of molecular oxygen in the test environment increased the adsorptive capacity of activated carbon for background organic matter, yielding a delay in its breakthrough from an adsorber. Because of the increased capacity for background organic matter, the resultant capacity for volatile organic chemicals decreased because of competitive effects.