The electrochemical characterization of both the mono- and di-substituted forms of ferrocene sulfonate are given. The results show both species produce voltammograms consistent with quasi-irreversible diffusion controlled redox reactions. The FcSO(3)(-) species was found to be easier to oxidize than its Fc(SO3)(2)(2-) counterpart, due to the electron withdrawing affect of the sulfonate group on the Fe centre. In the presence of sulfide, the voltammetric response of FcSO(3)(-) is shown to be consistent with the occurrence of an electrocatalytic EC' reaction. This analytical response was utilized as a means of determining sulfide and was found to be linear over the concentration 0.02-1 mM with a limit of detection of 14 mu M. (c) 2006 Elsevier Ltd. All rights reserved.