Capacitance-potential measurements by ac impedance methods have shown that for polarisations of 0.5 to 0.7 V from the flat band potential, titanium passivation films formed between 0.2 and 2 V (thicknesses ranging from 15 to 100 Angstrom) display Mott-Schottky behaviour, with frequency dependent slopes. This study develops a theoretical approach which takes account of the frequency dispersion and allows the determination of flat band potentials, donor densities, and Helmholtz capacitances for the films. The Mott-Schottky behaviour was attributed to the presence of Ti3+ as the doping element. Experimental evidence for this was obtained from XPS spectra. For films formed above 2 V the linear Mott-Schottky region was difficult to define, and capacitance vs. potential plots resembled the behaviour of amorphous semiconductors.