The Langmuir adsorption isotherm equation is extended to include a third parameter. The extended Langmuir model is then compared with the Aranovich-Donohue (A-D) and the n-layers BET models in their ability to fit experimental data. The extended Langmuir model shows the smallest deviation of all the three-parameter models discussed, and has a smaller deviation than certain of four-parameter models. Unlike the models that have a singularity when pressure equal to saturation vapor pressure, the extended Langmuir model performs well in numerical calculation as well as its ability to fit experimental data. (C) 2009 Elsevier B.V. All rights reserved.