The chromatographic behaviour in the reversed-phase mode of various drug substances, which are nitrogen-containing bases, has been investigated. Three different types of column materials were used, ordinary chemically bonded ODS-silica, deactivated ODS-silica specifically designed for the separation of bases and bare silica dynamically modified by adding long-chain quaternary ammonium compounds to the eluent. Various brands were tested for all three types of column materials. When using ODS-silica as the column material, considerable tailing of peaks and prolonged retention was observed. Also the deactivated columns exhibited tailing problems, but less pronounced. For both types of bonded-phase material it was possible to reduce the tailing considerably by the addition of an anti-tailing agent to the eluent. On bare silica used in the dynamic modification mode the tailing was almost negligible.