The article is concerned with a simple method for preparing cationic resins from polycondensation of Egyptian bagasse pith (as a source of cheap and renewable material) with phenol and paraformaldehyde as a cross-linking agent. Optimum principal reaction conditions of the preparation and properties are determined and compared with resin without bagasse pith content. The synthesized resins are stable in water, organic solvents, thermal treatment, and mineral acids (1 M). The samples having a cation exchange capacity up to 3.92 meq g-1 of dry resins are being introduced as new cationic exchangers. The synthesized resins are used in the study of the possible separation of univalent cations. The rational thermodynamic equilibrium constants (In K) are calculated for Li+-Na+ exchanges on the resins having a various amount of bagasse pith. The thermodynamic parameters are computed and suitable explanations are described.