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Targeted α-therapy (TAT) can eradicate tumor metastases while limiting overall toxicity. One of the most promising α-particle emitters is astatine-211 (²¹¹At). However, ²¹¹At-carbon bonds are notoriously unstable in vivo and no chelators are available. This hampers its adoption in TAT. In this study, the stability of ²¹¹At on the surface of gold nanoparticles (AuNPs) was investigated. The employed AuNPs had sizes in the 25–50 nm range. Radiolabeling by non-specific surface-adsorption in >99% radiochemical yield was achieved by mixing ²¹¹At and AuNPs both before and after polyethylene glycol (PEG) coating. The resulting ²¹¹At-AuNPs were first challenged by harsh oxidation with sodium hypochlorite, removing roughly 50% of the attached ²¹¹At. Second, incubation in mouse serum followed by a customized stability test, showed a stability of >95% after 4 h in serum. This high stability was further confirmed in an in vivo study, with comparison to a control group of free ²¹¹At. The AuNP-associated ²¹¹At showed low uptake in stomach and thyroid, which are hallmark organs of uptake of free ²¹¹At, combined with long circulation and high liver and spleen uptake, consistent with nanoparticle biodistribution. These results support that gold surface-adsorbed ²¹¹At has high biological stability and is a potentially useful delivery system in TAT.