The object of this research was to study trace contaminants coadsorption phenomena on activated charcoal. Selected compounds were injected in pulses into a stream of nitrogen carrier gas flowing through a packed column of activated charcoal. The effluent concentration of the injected compound was detected by either a flame ionization detector or an electron capture detector. The first and second moments of the effluent peaks of injected compounds were calculated and the observed changes in these moments were then interpreted in terms of changes in the adsorption equilibrium constant (H) and the mass transfer resistance coefficient (1/fl). Adsorption studies of the following compounds were done: (1) methanol at 1°C, (2) 2-propanol at 175°C, (3) ethyl acetate at 25°C, (4) trichloroethylene at 25°C and 29°C, (5) 1,1,1-trichloroethane at 2°C and 22°C, and (6) toluene. For each of the first five compounds, the following cases were considered: adsorption on (a) a clean activated charcoal bed, (b) a bed having the same compound preadsorbed on it, (c) a bed having toluene preadsorbed on it, and (d) a bed having toluene and the same compound, both preadsorbed on it. Methanol, 2-propanol and ethyl acetate were studied with a FID. Each of these three compounds were irreversibly adsorbed to some extent. The presence of the irreversibly adsorbed portion and toluene decreased the first and second moments of subsequent adsorption peaks. Changes in both moments indicate that the adsorption equilibrium constant decreases as the surface coverage by toluene and the irreversibly adsorbed amount increases. In the case of 1,1,1-trichloroethane and trichloroethylene, a substantial amount of irreversibly adsorption was noticed. The irreversibly adsorbed amount in each case showed signs of forming a pseudo-liquid layer on the activated charcoal surface. The effect of this was an increase in both moments. Toluene, however, decreased both moments of these two compounds. Sample doses in the experiment were kept low enough to get an average surface coverage of the order of 1“J to 1-4. However, the actual surface coverage at some sites might have been much higher resulting in a highly non-ideal behavior as was observed with 1,1,1-trichloroethane and trichloroethylene.