The structural characteristics of titanium nitride films deposited by d.c. reactive magnetron sputtering on (001) silicon wafers were studied by cross-section transmission electron microscopy and in-situ spectroscopic ellipsometry (SE). The growth habit of the films, the shape and the size of the crystallites were investigated versus deposition temperature, bias voltage and nitrogen flow rate. The surface micro-roughness was also studied by atomic force microscopy. The films exhibit a columnar growth showing preferred orientation along (111) direction. High-resolution transmission electron microscopy (TEM) observations reveal that the Si substrate does not affect the mode of growth. The SE measurements and TEM observations strongly suggest that TiN rich in nitrogen is formed at nitrogen flow rates in the range 1.9-3 seem. At a nitrogen how rate F-N= 1.7 seem the stoichiometric compound Ti2N is formed which exhibits a completely different mode of growth. The ability of the in-situ SE to monitor small structural changes induced by small changes of the growth conditions was tested by the deposition of sequential layers. In this respect SE is a powerful method to control TiN deposition.