The change of double-layer capacitance (C-dl) behaviour of polycrystalline Pt electrodes upon controlled formation of thin oxide films is evaluated. The capacitance behaviour of the oxide film interface with aqueous H2SO4 and HClO4 is quite different from that at the unoxidized Pt surface, while the presence of Cl- ion lowers C-dl of the latter interface. This indicates that extrapolation of C-dl data for the so-called `double-layer region' into potential ranges where an oxide film exists, would be a quite unreliable basis for double-layer corrections to recorded transient data. The presence of Cl- or Br- ions causes appreciable lowering of C-dl until onset of surface oxidation. By means of sensitive EQCN nanogravimetry measurements, the competitive effects of Cl- or Br- adsorption on oxide film formation at Pt can be quantitatively evaluated. Interestingly, adsorption of Cl- leads to net negative mass changes while Br- causes net positive ones over the oxide region. These results are interpreted in terms of specific mass changes due to diminution of O coverage concomitant with increasing halide coverage as a function of potential and Cl- or Br- concentration. (C) 2003 Elsevier Ltd. All rights reserved.