This paper deals with the study and the control of target poisoning by reactive gas in reactive planar magnetron cathodic sputtering. It is applied to the realization of TiN thin films. Using an original XPS analysis both of the target and of the TiN films, and an emission spectroscopy analysis of the discharge, the nitriding mechanisms of the target were analysed. We have shown that the target poisoning controls, in great part, the reactive phenomenon on the substrate and therefore the coating properties. Furthermore the TiN molecule has been detected in the target-substrate transfer space. The necessity to monitor the target poisoning rigorously to monitor the coating process is pointed out. Control by optical emission spectroscopy seems to be a good technique to monitor the coating growth kinetics and the coating properties.