The structural attributes and molecular interactions contributing to "U-shape" retention on pentafluorophenylpropyl (PFPP) HPLC stationary phases are systematically investigated. Only basic analytes exhibit retention that increases with the acetonitrile content in mixtures of acetonitrile and aqueous ammonium acetate, with some basic analytes not eluting at all from PFPP columns using 100% acetonitrile. U-shaped retention as a function of mobile phase acetonitrile content was more dramatic on a PFPP column relative to C18. Retention of the quaternary ammonium salt bretylium on these stationary phases and on the same bare silica support showed minimal influence of ion-exchange mechanisms on the C18 phase, however, a significant influence of ion-exchange mechanisms was observed for both PFPP and bare silica. The retention of bretylium on PFPP was only slightly less than on bare silica. These findings suggest ion-exchange mechanisms dominate retention of basic analytes in the high acetonitrile realm on PFPP. The PFPP stationary phase exhibits a substantial increase in effects of ionized surface silanol groups compared to the alkyl phase despite similar surface coverage. Retention of some basic analytes on a PFPP phase was enhanced relative to retention on silica alone, and implicates other dispersive interactions that might be exploited for selectivity different from either alkyl phases or silica alone.