The use of a membrane in gas separation is gaining popularity and is considered to be the most prominent emerging technology in gas separation. In order to determine the effectiveness of a membrane in separating gases, it is important to have an accurate estimate of the pore size distribution. This article presents a method to estimate the pore size distribution from adsorption isotherm data. Adsorption isotherm data are used to determine the pore size distribution using both slit and cylindrical models. The theory that relates adsorption isotherm to pore sire distribution is presented. The method is validated with experimental data and it is shown that both the models exhibit narrow distribution profiles.