Ionic interactions have been determined from the experimental osmotic coefficients, densities, and sound speeds for the mixing of guanidinium cation (Gn(+)) with K+, tetra-n-butylammonium cation (nBu(4))N+, Mg2+, and Ca2+ in aqueous solutions containing Cl- at several ionic strengths. As Mg2+ and Ca2+ are more hydrophilic than K+, and (nBu(4))N+ is a strong hydrophobic cation, their mixing with another strong hydrophobic species, that is, Gn(+), throws light on the importance of binary and ternary interactions involving two cations and one common anion. The excess free energy of mixing, Delta (m)G(E), volumes, Delta mV(E), and compressibility, Delta mK(E) calculated from independent osmotic coefficient, density, and sound speed measurements indicate strong binary interactions as a result of mixing of K+-Gn(+) and (nBu(4))N+-Gn(+) cation pairs. The mixing of Mg2+ with Gn(+) and Ca2+ with Gn(+) suggests the neutralization of opposing effects produced by the hydrophilic and hydrophobic cations. This situation, which is similar to that noted in an earlier study (Kumar, A. J. Phys. Chem. B 2000, 104, 9505) on the mixing of Na+ and Gn(+), offers important information on the binary and ternary interactions present on the mixing of one hydrophilic or hydrophobic cation with another hydrophobic cationic species.