The effects of chloride and sulfate salts of tetrapropylammonium (TPA+) and guanidinium (Gdm+) on the conformational stabilities of tryptophan zipper (trpzip) and .-helical peptides (alahel) were measured by circular dichroism spectroscopy. Like Gdm+, TPA+ interacts with the planar tryptophan indole group perturbing the conformational stability of trpzip peptides. TPA+ effects are largely unaffected by sulfate indicating an absence of the hetero-ion pairing that is observed in concentrated Gdm+2SO42- solutions. TPA+ strongly stabilises helical conformations in alahel peptides indicating exclusion from the peptide bond. The observations are broadly consistent with predictions from molecular dynamics simulations [Mason, P. E. et al. J. Phys. Chem. B 2009 113, 3227-3234], indicating that the effects of complex ions on proteins are increasingly predictable in terms of ion hydration, complementary interactions with specific protein groups, and ion pairing contributions.