Chonoamperometry of ferricene in acetonitrile containing a small amount of supporting electrolyte was studied to verify the theoretical model developed by Hyk et al. (W. Hyk, M. Palys and Z. Stojek, J. Electroanal. Chem., 415 (1996) 13). Some minor extensions were made in this model to adjust it better to the case studied. The experimental chronoamperograms were found to agree well with the theoretical predictions. The biggest deviations were observed for small support ratios. These deviations can be explained by the uncontrolled presence of ionic impurities in the solvent. The impurities influence significantly the conductivity of the solution for support ratios (ratio of concentration of supporting electrolyte and ferrocene) smaller than 0.00075, and cannot be included explicitly in the theoretical model. The influence of the geometry of the electrode and the effect of the double layer capacitance are discussed. A possibility of evaluating the total ion level and the formal potential of the system investigated, by fitting an experimental curve to the theoretical one, is also presented.