The study of mixed Al3+/Fe3+ nitrate hydrates is gaining increasing interest with the emergence of new hydraulic binders and Supplementary Cementitious Materials (SCMs) containing significant amounts of iron and aluminum oxides that can be activated by nitrate addition. Layered AFm-NO3 hydrates containing varying proportions of trivalent Al3+ and Fe3+ cations were synthesized and characterized. The AFm-NO3 phase constitutes a continuous solid solution between the two end-members Ca2Al(OH)6·NO3·2H2O and Ca2Fe(OH)6·NO3·2H2O. The obtained Ca2Al1-xFex(OH)6·NO3·yH2O solid solution proves stable for four hydration states, with a significant impact on the interlayer distance: y = 5 (at high relative humidity with an interlayer distance of 10.4 Å), y = 2 (at ambient conditions with an interlayer distance of 8.6 Å), y = 1 (above 60 °C with an interlayer distance of 7.4 Å) and y = 0 (anhydrous above 175 °C with an interlayer distance of 8.0 Å). In fact, thermal variations reveal some differences between the two Al-rich and Fe-rich sides. Finally, AFm-NO3 stability was examined in the presence of other anions, demonstrating the low stability of NO3− with respect to Cl− and CO32− anions.