A series of nine compounds containing quaternary ammonium and substituted pyridinium cations and the bis(4-methylbenzene-1, 2-dithiolato)cobaltate(III) complex anion have been prepared and their ion-pair properties studied by lH n.m.r. spectroscopy. Their concentration association constants in nitrobenzene at 307 K range from 9 to 20. In the pyridinium series of ion pairs, it is concluded from the measured isotropic shift ratios that methyl substituents at the meta and para positions favour a geometry where the anion is tilted towards the ring of the cation but the ortho substituents push the anion away from the ring. In the methyltrioctylammonium ion pair, there is linear correlation between the observed isotropic shift of the N-methyl or the N-methylene protons and the dielectric constant of the solvents (ε) from 4 · 56 to 12 · 3 for the 0 · 06, 0 · 09 and 0 · 12 m solutions. Its implication is discussed in terms of the formation of the ionic clusters of sufficiently large size.