The ferrocene-ferricinium electrode (Pt/Foc. Fit') was investigated in water, acetonitrile, ethanol. DMSO and DMF using single scan cyclic voltammetry and phase sensitive ac polarography. The oxidation-reduction is pseudo-reversible in all five solvents with an electrochemical rate constant of approximately IO-' cm/s. In all solvents a slow Irreversible chemical step involving the ferricinium cation follows electron transfer, so that slow cyclicrvoltammetry or polarography rather than potentiometry is preferred if ferrocene is to be used as a reference electrode in non-aqueous solvents. The Strehlow assumption. AG,,(Foc) = AG,,(Fic') gives very different free energies of transfer of single ions from non-aqueous solvents to water &hen compared with the TATB assumption that AG,,(Ph,As' )= AG,,(Ph,B-). This discrepancy is likely to be because ferricinium is only a moderately large cation, so that AG,,(Fic') is less positive than AG,,(Foc) for transfer to water. The discrepancy is not because of abnormal electrochemical behavior of the Pt/Foc, Fit' electrode in water or other solvents. Values of E L'S U!W. H,O in a variety of solvents based on the TATB assumption are presented.