This application note highlights the characteristics of a new mixed-mode reversed-phase/anion-exchange stationary phase that offers very different selectivity compared to traditional RP stationary phases. Atlantis BEH C18 AX has excellent batch-to-batch reproducibility and exceptional high-pH stability relative to existing mixed-mode RP/AX products. Compared to conventional RP stationary phases, when used with mobile phase pH values less than about 8, Atlantis BEH C18 shows increased retention of anions, such as ionized acids, and decreased retention of cations, such as protonated bases. The selectivity for ionizable analytes may be varied over a wide range by adjusting the pH and buffer concentration of the mobile phase. When using low ionic strength mobile phases (e.g., containing 0.1% formic acid), Atlantis BEH C18 AX shows extraordinary sharp and symmetric peaks for protonated bases. This new stationary phase is complementary to existing RP stationary phases, making it useful for column screening during method development.