AbstractThe nitrogen and carbon activities are the cardinal parameters for process control of nitriding, nitrocarburising, carburising, and carbonitriding. The essential elements of the thermodynamic background for the definition of the so called nitrogen and carbon activities in nitrided and carburised surface layers of iron based substrates are presented in a comparative manner. Choice of the distinct reference states is discussed in relation to the consequences for the interpretation of activities and equilibrium constants of the reactions. Practical examples are used to show how the nitrogen and carbon activities can be established; the activities can, for example, be used to determine the amounts of dissolved nitrogen and carbon. Also, knowledge of the nitrogen and carbon activities makes it possible to minimise and to control nitrogen gas pore formation and/or to avoid cementite formation in engineered surface layers.