A model of Langmuir monolayer liquid adsorption into effective pores was used to study the monolayer adsorption capacity by theory calculation. The activated carbon (AC) from rice husk with NaOH activation was selected as adsorbent to uptake phenol and methylene blue (MB). Materials were characterized by N-2 adsorption, infrared spectroscopy (IR), and ultraviolet spectrophotometer (UV). In adsorption kinetics, it was observed that the experimental data were well explained by the pseudo second-order equation. Moreover, the Langmuir isotherm was more suitable to explicate results than Freundlich isotherm, implying the monolayer adsorption. Basing on the molecule diameter of 0.72 nm and cross-sectional area of 0.414 nm(2) for phenol, the theoretical adsorption capacities were close to the actual values with small relative error (<7%). Due to the large molecule size of MB (0.98 nm, 0.749 nm(2)), the sample with wider pores activated at 900 degrees C exhibited the higher adsorption capacity than AC at 700 degrees C. And the capillary condensation increased the adsorption capacities, consistent with the order of average pore diameter for ACs. From the model, it indicated that the adsorbent was suitable for adsorption when its critical pore width was two times of adsorbate molecule diameter. (C) 2012 Elsevier Inc. All rights reserved.