In aqueous solution, the tetrakis(ethylisocyanide)platinum(II) cation, Pt(CNC2H5)(4)(2+), reacts with tetracyanoplatinate(II), Pt(CN)(4)(2-), to form four multinuclear species. These species are the diplatinum(II) compound [Pt(CNC2H5)(4)(2+)][Pt(CN)(4)(2-)], two triplatinum compounds, [Pt(CNC2H5)(4)(2+)](2)[Pt(CN)(4)(2-)](2+) and [Pt(CNC2H5)(4)(2+)][Pt(CN)(4)(2-)](2)(2-), and the tetraplatinum species [Pt(CNC2H5)(4)(2+)](2)[Pt(CN)(4)(2-)](2). The interactions leading to the stability of these species are not simply ionic. The interactions result in major differences in the electronic absorption spectra, and the oligomers are not disrupted by the addition of other salts. The formation constant for each of the oligomers was determined at several temperatures, and the enthalpies and entropies of formation were measured. Species with even numbers of platinum ions have more negative stepwise enthalpies of formation than do the species with odd numbers of platinum ions. Entropies of formation are negative as well; however, the values are significantly more positive for the species with odd numbers of platinum ions than for those with an even number. These thermodynamic trends can be rationalized by recognizing that the species with even numbers of platinum ions are dipolar and organize the solvent better than the species with odd numbers of platinum ions, which are not dipolar. Thus, the more negative enthalpies of formation of the species with even numbers of platinum ions are a consequence of better solvation than species with odd numbers of platinum ions. Accordingly, the entropies of formation of the species with even numbers of platinum ions are more negative because they organize solvent to a greater extent. Exclusive of solvent effects, the anion-cation interaction is about 30 kJ/mol.