Spectroscopic ellipsometry (SE) was employed to investigate the growth of atomic layer deposited (ALD) TiN thin films from titanium chloride and ammonia and the followed oxidation in dry oxygen. Two regimes were found in the growth including a transient stage prior to a linear regime. The complementary ex situ characterization techniques showed a good agreement with the results obtained from SE measurements. A columnar structure of the as-deposited TiN film, which was composed of grains surrounded by amorphous material in between, was obtained. The X-ray photoelectron spectroscopy (XPS) analyses indicated low chlorine impurity content and slightly N-rich TiN films. The existence of an intermixed layer between the nitride and oxide during the oxidation was verified by the XPS depth profile analysis for a partially oxidized TiN film. A three-layer optical model was constructed for SE in situ monitoring the oxidation. A four-regime oxidation was found for TiN films whereas only two regimes were seen in the case of films. A new oxidation mechanism was proposed to explain the oxidation behavior of thin TiN films.