The stabilized Au NPs exhibit very high catalytic activity in the selective reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH4 as a reducing agent (Fig. ), a widely used model reaction for evaluating the catalytic activity of nanocatalysts in aqueous media. Moreover, they show robust stability against agglomeration and excellent dispersibility for at least 2 months, as well as excellent accessibility in solution, as demonstrated by turnover frequencies superior to "naked" Au nanoparticles stabilized solely by excess BH4 -anions. The stabilization effect of K,Na-PHI is elucidated using a comparative analysis employing different structural units of K,Na-PHI (cyanurate, melonate, and cyamelurate) and ab initio theoretical calculations, and the dominant role of functionalized heptazine moieties for the stabilization of Au NPs is discussed.