A series of compounds containing substituted pyridinium cations and the tribromo(triphenyl-phosphine)cobaltate(II) complex anion have been prepared and studied by n.m.r. spectroscopy in nitrobenzene at 307 K. It is shown that the electronic and the steric effects of the substituents can affect the ion-association constant. From the measured isotropic shift ratios, it is concluded that two substituents at the meta positions of the pyridine ring move the complex anion away from the ring, whereas a para substituent tends to favour a geometry where the anion is tilted towards the ring.