Sulfonic acid-functionalized monolayer-protected gold clusters (SO3-MPCs) have been synthesized by the reaction of the Bunte (thiosulfate) salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (SO3) with tetrachloroaurate and borohydride in aqueous acetic acid. The nanoparticles produced have an average core diameter, obtained by transmission electron microscopy (TEM), of 2.2 ± 1.1 nm, and a 30 wt % ligand content, obtained by thermogravimetric analysis. These parameters, assuming an ideal closed-shell truncated octahedral nanoparticle, correspond to an average nanoparticle formula of Au₃₁₄SO3₁₁₁. Proton NMR, Fourier transform infrared (FTIR), and UV/vis spectroscopies are consistent with nanoparticle formation, and acid/base titrations are consistent with the strong acid character of the SO3-MPC monolayer. These nanoparticles add to a growing family of water-soluble MPCs. Synthesis of a salt of SO3-MPC with a polyether-tailed triethylammonium countercation yields an ionically conductive molten salt, which is studied by alternating current impedance spectroscopy in the solid state (neat melt) and solution form.