Low alloy AISI 4140 steel was ion nitrided at temperatures of 500, 550 and 600°C for various times in a 50%N 2 -50%H 2 gas mixture. The nature of the compound layer formed was studied by X-ray diffraction and optical metallography. The X-ray analysis shows the existence of a γ-Fe 4 N compound zone on the surface at all three temperatures. The thickness of the compound layer increases with increasing time at 500 and 550°C; however, it begins to decrease with increasing time at 600°C.