The passivity of titanium was studied using potentiostatic polarization combined with Mott-Schottky analysis in acidic solutions. The oxide layer was characterized as an n-type semiconducting, oxygen deficient oxide (TiO1.993-1.996) with a donor density in the range of 10(19)-10(20) cm(-3) depending on electrode potential and electrolyte pH. The calculated thickness for the inner oxide layer was in the range of 1-4 nm, increasing linearly with applied potential and exponentially with electrolyte pH. The potential- and pH-dependence of the inner oxide thickness was interpreted by a modified point defect model for the migration-controlled oxide growth, in which the rate-determining step in the passive film growth processes was assumed to be the donor lattice migration. (C) 2010 Elsevier BM. All rights reserved.