Polyaniline, poly(2-methoxyaniline) and poly(2,5-dimethoxyaniline) were electrochemically deposited on gold electrodes modified with self-assembled alkanethiol monolayers. The thiol-polymer layers obtained were investigated by reflection-absorption IR spectroscopy and cyclic voltammetry in the presence of redox probes: hexacyanoferrate (II) and hexaamminoruthenium (III). It was found that the layers exhibit an increased ability of mediation in electron transport between the electrode metal and redox species present in the solution, when the polymer is in its oxidised form. On the contrary, they reveal passivating properties when the polymer is in the reduced, non-conducting state. Although the switching between the conducting and non-conducting form is a fundamental property of conjugated polymers, it can be effectively exploited only for very thin films of high integrity. The condition is fulfilled in the case of 'mixed' thiol-polymer layers, and this makes them promising in view of possible analytical applications.