Two quaternized pyridinium compounds being considered for application as corrosion inhibitors, N-butyl-3-(hydroxynonyl) pyridinium bromide (Quat 1) and N-butyl-3-(N-octylcarbamyl)pyridinium bromide (Quat 2), differing only in the nature of the linkage between the pyridinium ring and the group attached at the meta position, have been investigated for their adsorptive properties at polycrystalline Pt electrodes in pH 7 and 11 solutions. It is seen that both Quats are strongly adsorbing, even at a concentration of < 50 muM, completely blocking the Pt surface and fully suppressing both Pt electrochemistry and hydrogen evolution. In situ mass measurements, carried out together with electrochemistry, reveal that one to two monolayers of Quats 1 and 2 are deposited when cycling between the potentials of oxygen and hydrogen evolution. Notably, once the Pt surface and its electrochemistry are fully blocked, the potential can be extended more negatively, allowing the reduction of Quats in solution, forming a multilayer film on the Pt surface. Concurrent mass measurements show that the equivalent of up to 100 monolayers of reduced Quat can then be deposited, probably as a mixture of monomers and dimers. (C) 2003 Elsevier Science B.V. All rights reserved.