Aminopyrazole is a hydrophilic basic compound used as a building block in manufacturing of drugs. This basic compound can be retained by weak reversed-phase and strong cation-exchange mechanism on Primesep bimodal HPLC columns. Retention time is primarily controlled by cation exchange interaction and the amount of ions in the mobile phase. Comparison of Primesep A, Primesep 100 and Primesep 200 shows relative strength of each column towards retention of basic compounds.