The electron paramagnetic resonance (EPR) of Mn²⁺ was used to determine pore size distribution of silica gel and aerosil200. The paramagnetic probe Mn²⁺ was chosen because it is sensitive to its environment. In presence of water molecules, [Mn(H₂O)₆]²⁺ paramagnetic species can be formed. Consequently, all of the changes of water pressure lead to modify the EPR spectra of Mn²⁺. Using Kelvin law, a pore radius distribution can be established. By this EPR technique, the pores radius values of silica gel and aerosil200 are 5Å and 16Å respectively. The comparison between the results obtained from the EPR and BET techniques is satisfactory and seems promising. EPR technique was tried to apply to cotton fibers.