The authors investigate the validity of the rotational distribution of N₂⁺(B) to N₂⁺(X) emissions as a tool to determine the substrate temperature in surface treatment reactors. They show that the method is not valid for processes involving pure nitrogen at p>1.3*10² Pa. In N₂-H₂ mixtures, however, they show that the technique can be used for temperature measurements for all pressure intervals normally used in ion nitriding treatments. This discrepancy is interpreted in terms of excitation paths of the N₂⁺(B) state in the plasma reactor.