The accumulation of reports on the puzzling behavior of guanidinium-rich oligo/polymers in bilayer membranes, reaching from HIV-Tat-like (HIV Tat is the human immunodeficiency virus transactivator of transcription) translocation to selectivity and voltage-gating of ion channels, prompted us to investigate possible contributions from counteranions to these phenomena. We report that anion-mediated variability of charge and solubility makes guanidinium-rich oligo/polymers adaptable to many environments. For example, poly- and hexaarginine but not polylysine phase transferred from water into chloroform in the presence of amphiphilic anions such as monomeric sodium dodecyl sulfate (SDS), egg yolk phosphatidylglycerol (EYPG), cholesterol sulfate, pyrenebutyrate, and stearate. Hydrophilic anions with high affinity inhibited phase transfer of 5(6)-carboxyfluorescein (CF)-polyarginine complexes into bulk membranes (sulfate, adenosine 5‘-triphosphate, adenosine 5‘-monophosphate, heparin, and micellar SDS). At least binary anion cocktails were necessary to activate polyarginine as a carrier in bulk chloroform membranes. Refined combinations of EYPG, phosphate, and azide or TFA were found to maximize translocation of CF across bulk membranes by polyarginine. Polyarginine-mediated CF efflux from large unilamellar vesicles was best in the presence of EYPG in the bilayer as well as phosphate and TFA in the medium. Similar regulatory activities of several anions were in support of a common carrier mechanism for guanidinium-rich oligo/polymers in bulk and bilayer membranes. The identified activities of polyarginine in bulk and lipid membranes suggested that anion-mediated adaptability of the solubility of guanidinium-rich oligo/polymers cannot be ignored in studies on biological function. The infinite variability and dynamic nature of available regulatory anion cocktails may contribute to the elusive character of guanidinium-rich oligo/polymer function in biomembranes.