The positron lifetime technique has been used for studying the defect structure of titanium (Ti) thin films obtained by d.c. magnetron sputtering on Ti substrates. It was found that for non-annealed samples large voids exist in the Ti films. Sample annealing for 1 h at 450 degrees C leads to a decrease of the void concentration. By checking the linearity of the valence annihilation parameter S versus the core annihilation parameter W we have shown that the type of the defects in the films of different thickness is the same. Comparison of the characteristic slope R(d) for non-annealed and annealed samples demonstrates that annealing changes the defect types in the films.