Radical cations and dications of three bishydrazines belonging to the Class II mixed valence compounds have been generated, either spontaneously or by oxidation with AgSbF6, in two 1-butyl-3-methylimidazolium (4(+)) ionic liquids having BF4- and PF6-as counterions. The optical spectra of these intermediates have allowed evaluation of Marcus' reorganization energy lambda(s), a parameter that is directly proportional to the solvent polarity. Remarkable differences in lambda(s), as large as 600 cm(-1), have been observed as a function of the counterion, with these data providing support for the observed differences between both ionic liquids (4(+)BF4(-) and 4(+)PF6(-)) in catalysis. However, in terms of polarity, the lambda(s) values rank the hydrophilic 4(+)BF(4)(-) as being similar to dimethyl sulfoxide and dimethylformamide, while the polarity of hydrophobic 4(+)PF(6)(-) is analogous to acetonitrile. Overall, our results indicate that ionic liquids are not exceptional liquids in terms of polarity.