This paper deals with ion-nitride processing and is primarily concerned with the effects of carbon content in the workpiece and of added carbon in the gas atmosphere on hardness, compound layer thickness, and diffusion layer thickness. Increased carbon content in the workpiece increases the compound layer thickness, but decreases the diffusion layer thickness. On the other hand, an optimal amount of added carbon in the gas atmosphere will increase the compound layer thickness as well as the diffusion layer thickness and hardness. It is found that the ion-nitriding temperature at which maximum hardness is attained is governed by the ion-nitriding time. When the treatment time is made sufficiently long, the maximum hardness is obtained at a temperature near 450°C.