The oscillation process observed on iron in sulphuric acid at the beginning of the dissolution plateau has been studied by measuring simultaneously the electrolyte resistance and the current vs time. The influence of the viscosity and mass transport has been investigated by using a rotating disc electrode (RDE), a submerged impinging jet (SIJ) cell and a rotating cylindrical electrode (RCE). The oscillation process occurring at the beginning of the plateau could be linked to the existence of a viscous layer of low conductivity corresponding to a colloidal dispersion of salt or/and hydroxide.