The paper gives results of an examination of the performance characteristics of activated carbon as a gas-phase air cleaner medium under various levels of relative humidity (RH) and a range of toluene concentrations. Toluene breakthrough curves at levels of humidity from below 5 to 80% were obtained. The concentration of the toluene challenge varied from 0.4 to 72.8 ppm. The experiments were performed in a small-scale test rig with temperature, flowrate, and humidity controls. These experiments provided data to explore the relationship of useful lifetime to RH. Below 50% RH, RH appears to have negligible influence on the adsorption of toluene but, by 75% RH, the toluene adsorption is decreased. Above the inflection point in the relationship between the cabon water content and the RH of the challenge air, the water interferes with the adsorption of toluene. Thus, the water adsorption curve supports the toluene adsorption data. The lineaarity of the relationship between the concentration and the 10% breakthrough time indicates that high concentration breakthrough data can be used to predict the breakthrough time for lower concentration challenges.