Electrochemical impedance spectroscopy has been used for the characterisation of electrodes modified with different polythiophenes, namely poly[4,4 ' -bis(butylsulfanyl)-2,2 ' -bithiophene], poly[4,4 ' -bis(methylsulfanyl)-2,2 ' -bithiophene] and poly( 3-methylthiophene), at different applied potentials, using different supporting electrolytes. By comparison of the results obtained under experimental conditions in which n-doping is prevented and those obtained from tests where it does occur, some general features have been deduced. all of them being coherently described by a recently proposed 'generalised transmission line circuit' model: impedance plots at different applied potentials exhibit progressive changes which are well accounted for by the 'evolving' model. The results obtained on the n-doping process of S-alkyl substituted polymers suggest a behaviour interestingly similar to that exhibited in the p-doping: this supports a symmetry that was also found by us in a previous work, with respect to the incorporation and release of counterions during the n- and p-charge-discharge processes.