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This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were U 0.75 Pu 0.20 Am 0.05 O 2 , U 0.90 Am 0.10 O 2 and U 0.80 Am 0.20 O 2 . After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered U 0.80 Am 0.20 O 2 materials was investigated applying U M 4 and Am M 5 edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are U(iv)/U(v) and Am(iii)/Am(iv). The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for PuO 2 , AmO 2 , (U,Pu)O 2 or (U,Am)O 2 ., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)