Guanidinium (Gdm(+)) chloride is a powerful protein denaturant, whereas the sulfate dianion (SO42-) is a strong stabilizer of folded protein states; Gdm(2)SO(4) is effectively neutral in its effects on protein stability. While the "neutralizing" effects of protein-stabilizing solutes on the activity of denaturants can be broadly interpreted in terms of additive effects of the solutes, recent experimental and simulation studies support a role for hetero-ion interactions in the effect of sulfate on Gdm+ denaturation [Mason, P. E.; et al. J. Phys. Chem. B 2005, 109, 24185-24196]. Here we describe an experimental strategy for testing this mechanism that involves spectroscopic analysis of the separate effects of alkali metal sulfates (Na2SO4, Rb2SO4), GdmCl, and Gdm(2)SO(4) on the folded populations of several peptides chosen to dissect specific noncovalent contributions to the conformational stability of proteins [alanine-based helical peptides stabilized by hydrogen bonds, tryptophan zipper (trpzip) pepticles stabilized largely by cross-strand indole-indole interactions]. While the trpzip peptides are highly sensitive to GdmCl denaturation, they are unaffected by NaCl, Na2SO4, or GdM2SO4, indicating that the reversal of the denaturant activity of Gdm(+) by sulfate in this case is not due to competing stabilizing (sulfate) and destabilizing (Gdm+) interactions. GdM2SO4 was found to retain considerable denaturant activity against alanine-based a-helical pepticles. The differences in the effects of Gdm(2)SO(4) on the two peptide types can be understood in terms of the different mechanisms of Gdm+ denaturation of trpzip pepticles and helical pepticles, respectively, and the specific nature of Gdm+ and SO42- ionic "clustering" that differentially affects the ability of Gdm(+) to make the molecular interactions with the peptides that underlie its denaturant activity.