A kinetic study of the passive state of carbon steel in H2SO4 solutions was performed based on impedance measurements. Complex impedance diagrams with only two capacitive components were obtained in the potential range 0.6-1.4 V/SCE. The parameters of the passive electrode (R(HF), C-HF, alpha and R(m)) were obtained as a function of potential. The product IR(m) of the passive current I and the transport resistance R(m) in the passive film was found to be proportional to the film thickness. The thickness of the passive film increased with increasing polarization potential at relatively low potentials, but decreased with increasing degeneration of the outer layer of the film at high potentials. The polarization potential is lower than both trans-passive potential and the oxygen evolution potential.