The pore structure of expanded graphite (EG) including pore volume, pore size distribution and surface area is investigated by mercury porosimetry, nitrogen adsorption and SEM. Also, the diffusion of silicone oil molecules as macromolecules with different molecular weights into EG pores is studied. Various EG samples were prepared by the sudden heating of graphite intercalated compound (GIC) with varying particle size of 35, 50, 80 and 200 meshes in an electrical furnace at temperatures of 700, 800 and 900 °C. The EGs were characterized by FTIR to evaluate the presence of functional groups. It was found that the exfoliation process has not significantly introduced oxygen functional groups such as epoxy and carboxyl groups to the EG structure. Therefore the chemical structure of the EG is very close to pristine graphite. The mercury porosimetry results showed a broad range of total pore area from 5 to 31 m2/g for the EGs. The particle size of GIC and exfoliation temperature showed strong effects on the pore size of EG. The mercury intrusion porosimetry and nitrogen adsorption isotherms revealed that μm-pores are dominant as compared with nm-pores in all EG samples. The diffusion of silicone oils as macromolecular guests with three different viscosities was experimentally studied to analyze the diffusion facts of EG as the host. It was observed that as the exfoliation temperature decreases, the sorption capacity decreases; and EG samples prepared from the GICs with smaller particle size have lower sorption capacity. Sorption experiments also showed that the whole pore volume of EG is not filled with silicone oil leading to this fact that EG includes relatively closed pores. A new model was suggested for the sorption capacity of EG as a function of the pore area of EG and the square root of molecular weight of silicone oil.