The interaction of 4-(2-mercaptoethyl)pyridinium (HMEP+) hydrochloride with chromate was investigated using surface-enhanced Raman spectroscopy (SERS) and molecular modeling. It was shown that HMEP+ on the gold surface attracted chromate to the SERS surface and that the adsorption of chromate onto the substrate is described by a Frumkin isotherm. The ion-pair constant for the chromate-HMEP+ interaction is more than 3 orders of magnitude larger than that measured for the HMEP+ interaction with perchlorate ion. The high selectivity of HMEP+ for chromate is attributed to hydrogen bonding between chromate and the HMEP+ moieties on the SERS surface.