The electrochemical polymerisation of 5-carboxyindole in acetonitrile leads to the production of stable electroactive films. Forier-transform infared (FTIR) studies of the films are consistent with linkage of the monomer units through nitrogen and the 3-position of the ring. On transfer to aqueous electrolyte, These films show stable pH-dependent electrochemistry. Rotating-disc studies of the electrochemistry of [Fe(bpy)₃]^2+/3+ and [Fe(CN)₆]^3– allow us to identify the mechanism of the redox process in the each case. For [Fe(bpy)₃]^2+/3+ we find two redox processes, one corresponding to oxidation and reduction of species bound within the film and one corresponding to species diffusing through the film and reacting at the underlying platinum electrode. For [Fe(CN)₆]^3– reduction we find that the reaction occurs at the outside of the film. These results are consistent with a.c. impedance measurements and ex situ measurements of the conductivity which show that the films are insulating in their fully reduced or fully oxidised states but that they are conducting in their partially oxidised state. Based on FTIR studies, measurements of charge passed, and the pH dependence of the redox processes, structures are proposed for the different oxidation states at different solution pH.