This paper aims at characterizing the inorganic and organic pore size distribution (PSDinorg and PSDorg)of gas shale samples. To achieve this goal, we first perform nitrogen (N-2) sorption experiments using Brunuaer-Emmett-Teller (BET) analysis. N2 is an inert gas and hence, it condenses in both organic and inorganic pores. As a result, BET gives the total pore volume and PSD (i.e., PVtot and PSDtot). During water sorption (adsorption and desorption) process, water molecules can simultaneously be adsorbed by clays and condense in inorganic pores. To distinguish between the clay-adsorbed water and capillary-condensed water in inorganic pores, we use the modified water sorption technique (Zolfaghari et al., 2017). We calculate the inorganic pore volume and PSD (i.e., PVinorg and PSDinorg) from the modified sorption isotherms. By comparing the results of BET analysis and modified water sorption experiments, we obtain the organic pore volume and PSD (i.e., PVorg and PSDorg). The results suggest that the average organic and inorganic pore size is similar to 3 and similar to 10 nm, respectively. 2D visualizations of the shale samples using HIM-EDS (Helium Ion Microscope-Energy Dispersive Spectrometer) also suggest smaller organic pores compared with inorganic pores.