Perturbation of the passive-active transition state of the Fe \ 0.75 M H2SO4 system was carried out by adding a low concentration of halide ions (Cl-. Br-, I-). The dynamical response of the system to this chemical perturbation is studied by following changes observed in the I-E and I-t curves. The changes observed in I-E curves within the passive-active and passive regions are: (i) an extension of the oscillatory region beyond the Flade potential of the halide-free system, towards the stable passive region; (ii) a shift of the transpassivation potential towards lower values: (iii) an increase of the current in the passive region; (v) deviation of the maximum oscillatory current from the kinetics of the active legion. According to theses changes the aggressiveness of halide ions is classified in the order Cl- > Br- > I-. The main changes observed in potentiostatic I-t curves are: (i) existence of an induction period of time before the oscillations start; (ii) the oscillations deviate From the monoperiodicity observed for the halide-free system and they become complex periodic and aperiodic. The dynamical response of the halide-free Fe \ 0.75 M H2SO4, system is studied as a function of the applied potential, the concentration and nature of halide ions. On the basis of experimental results it seems that Cl- ions influence the stability of the oxide film and in particular the transition between an unstable in acid solutions oxide related to Fe3O4 and a more stable one related to Fe3O4 \ gamma-Fe2O3. Halide ions trigger local dissolution by an adsorption mechanism, which leads to the destabilization of the film related to gamma-Fe2O3 and establishment of appropriate electrochemical conditions for the lifting-off of the oxide and activation of the Fe electrode.