The Lehrer diagram often serves as a guide for selecting gas mixtures for nitriding alloy steels, but it is only accurate for ammonia gas nitriding processes when hydrogen is used as the diluting gas. This paper presents the results of a study showing that the use of pure nitrogen as a diluent has a marked effect on the phase boundary lines of the standard Lehrer diagram, essentially shifting them to the left. The paper also includes examples showing where the use of nitrogen is advantageous and where it is not.