The adsorption capacity of ZnCl2-impregnated activated carbon (AC) for NH3 is reported in terms of stoichiometric ratio of reaction (NH3 per ZnCl2). This ratio depends on the testing conditions used. Compared to the ratio obtained under dry conditions, the ratio is higher under humid conditions or increased NH3 concentrations. The linear increase of the NH3 capacity with increasing loading of ZnCl2 breaks down at about 3.5 mmol ZnCl2/g AC. This behavior is explained in terms of preferential adsorption of a monolayer of salt followed by aggregation of the impregnant once a monolayer is completed. The effect of increasing the loading of ZnCl2 on the capacity for gases for which the impregnants are not intended, namely cyclohexane, nitrogen, and water vapor, is also discussed. A break in the linear relationship between water capacity and impregnant loading at about 3.5 mmol ZnCl2 seems to correspond to a full monolayer coverage of ZnCl2 on AC. The monolayer of ZnCl2 is shown to reduce the uptake of water into AC, while the ZnCl2 aggregates are shown to be hydrophilic. (C) 2008 Elsevier Inc. All rights reserved.