The synthesis of a polypyrrole film on a non-conducting substrate is studied. A thin film of conducting polypyrrole is deposited on a non-conducting substrate by dipping the substrate in a pyrrole solution and subsequently in an oxidising solution containing Fe(ClO4)3 and HClO4. The influence of the acid concentration on the Fe3+ equilibria is calculated. The results of these calculations are in agreement with UV/VIS and pH measurements. The calculations show that for HClO4 concentrations of 0.50 M and higher the influence on the Fe3+ equilibria is negligible.