Reversed-phase (RP) is commonly used with low molecular weight compounds (amino acids, vitamins, drug substances, etc.) and peptides. It is popular in proteomics experiments where it is used to provide high-resolution peptide maps. Separation in RP is based on the analyte′s partitioning between a hydrophilic, aqueous mobile phase and a non-polar stationary phase. The common alkyl stationary phases (C18, C8, C5, C4) provide different retention, and selectivity can be imparted by using non-alkyl phases, like amide or phenyl. For biomolecule separations, the pore size of the support particle is an important consideration. Because retention can be strong and the mobile phase contain an organic modifier, RP is usually avoided with proteins that are needed to be recovered in their active forms. Tosoh Bioscience offers reversed-phase columns packed with silica or methacrylate particles. This section lists silica-based TSKgel reversed phase columns most suitable for biopolymer separations, such as TSKgel Protein C4-300, TMS-250 and OligaDNA RP, followed by columns designed for the analysis of small molecular mass compounds, such as TSKgel ODS-100V and ODS-100Z, and other established TSKgel RP columns.