We conducted a series of steady-state and short-term experiments on a three-phase circulating-bed biofilm reactor (CBBR) for removing toluene from gas streams. The goal was to investigate the effect of macroporous-carrier size (1-mm cubes versus 4-mm cubes) on CBBR performance over a wide range of oxygen loading. We hypothesized that the smaller biomass accumulation with I-mm carriers would minimize dissolved-oxygen (DO) limitation and improve toluene removal, particularly when the DO loading is constrained. The CBBR with 1-mm carriers overcame the performance limitation observed with the CBBR with 4-mm carriers: i.e., oxygen depletion inside the biofilm. The 1-mm carriers consistently gave superior removal of toluene and chemical oxygen-demand, and the advantage was greatest for the lowest oxygen loading and the greatest toluene loading. The 1-mm carriers achieved superior performance because they minimized the negative effects of oxygen depletion, while continuing to provide protection from excess biomass detachment and inhibition from toluene.