Using capillary electrophoresis and molecular dynamics simulations, we directly demonstrate that guanidinium cations exhibit an affinity for the positively charged side chains of arginine-containing peptides in aqueous solutions, while no such effect is observed for sodium cations and/or lysine. Such a counterintuitive pairing between these two positively charged moieties has implications for destabilization and aggregation of arginine-rich proteins in guanidinium salt solutions.