The N uptake of thin Ni foils was monitored by use of nitriding experiments performed in NH3-H2 gas atmospheres as function of the atmosphere’s composition between 673 K and 873 K (400 °C and 600 °C). With increasing nitriding potential $$ r_{\text{N}} = {{p_{{{\text{NH}}_{3} }} } \mathord{\left/ {\vphantom {{p_{{{\text{NH}}_{3} }} } {p_{{{\text{H}}_{2} }}^{3/2} }}} \right. \kern-0pt} {p_{{{\text{H}}_{2} }}^{3/2} }} $$ (with $$ p_{{{\text{NH}}_{3} }} $$ and $$ p_{{{\text{H}}_{2} }}^{{}} $$ being the partial pressures of the ammonia and hydrogen in the nitriding atmosphere at the Ni specimen’s surface) first an increasing uptake of N by the terminal Ni[N] solid solution was observed up to about 1 at. pct N. Upon surpassing a threshold nitriding potential, the Ni3N compound was formed. A thermodynamic interpretation of the observed data is only possible by taking into account the (small) degree of decomposition of the NH3 at the surface of the specimens, to arrive at the effective values of r N genuinely acting at the surface of the specimen. The data obtained were used to determine model descriptions for (i) the Gibbs energy of dissolution of N into Ni[N] (at infinite dilution), which was modeled as an ideal interstitial solid solution, and (ii) the Gibbs energy of formation of Ni3N (in equilibrium with Ni[N]). The thermodynamic model descriptions of Ni[N] and Ni3N, associated with these Gibbs energies, were used to calculate the Ni-rich part of the phase diagram Ni-N. A comparison of the values of the Gibbs energy of dissolution of N in various 3d-metal based terminal solid solutions reveals a considerable decrease of the stability of such solutions with increasing atomic number.