To develop a diffusion barrier layer for diamond deposition at high temperature onto steel, samples of steel coated with cathodic-arc-deposited and sputter-deposited titanium nitride were tempered under diamond-deposition conditions at temperatures up to 950 degrees C. Hardness of the steel and the adhesion were affected, no visible changes could be detected on sputtered TiN. Diamond was deposited by microwave plasma chemical vapour deposition onto steel samples coated with cathodic-arc-deposited TiN using substrate temperatures from 520 degrees C to 950 degrees C. The are-deposited TiN is not free enough from defects, so at temperatures >700 degrees C different reaction products can be observed and analyzed by electron-probe microanalysis. Depending on the temperature iron-chromium-carbon compounds and iron oxides can be seen, TiN reacts to become TiCxNy, and in some areas TiN delaminates.