In this paper the mechanism of SiO2 layer formation onto Ti3SiC2 and the propel-ties of the passive films formed on Ti3SiC2 and Ti4AlN3 in concentrated HCl solutions were investigated by SEM, polarization and EIS measurements. It is shown that the rate of the formation of a protective layer of SiO2 onto Ti3SiC2 surface during anodic oxidation is faster in the beginning, while after an induction period this rate is established at a value of about 130 mum cm(-2) per year, giving the rate of Ti dissolution of about 9 x 10(-4) moles cm(-2) per year. By analyzing the Mott-Schottky plots it was shown that the passive film onto Ti3SiC2 is an n-type semiconductor dominated by space-charge capacitance. Preliminary results for Ti4AlN3 indicate the presence of the passive film of a p-type semiconductor.