The synthesis of nanoscaled β-Bi₂O₃ starting from the bismuth oxido clusters [Bi₆O₄(OH)₄](NO₃)₆·H₂O, [Bi₂₂O₂₆(OSiMe₂^tBu)₁₄], [Bi₃₈O₄₅(NO₃)₂₀(DMSO)₂₈](NO₃)₄·4DMSO and [Bi₃₈O₄₅(OMc)₂₄(DMSO)₉]·2DMSO·7H₂O (OMc = O₂CC₃H₅) under ambient conditions is reported. The metal oxido clusters are regarded as ideal precursors for β-Bi₂O₃ due to their structural relationship with the latter. Nevertheless, different bismuth oxide polymorphs are accessible dependent on the hydrolysis protocol. Hydrolysis over a period of 18 h gave stable α-Bi₂O₃ whereas after 3 min an amorphous material is observed. Annealing of the amorphous material at 370 °C gave nanoscaled β-Bi₂O₃. An unusual high reactivity of the β-Bi₂O₃ particles with SiO₂ and Al₂O₃ is observed at temperatures above 400 °C. Powder X-ray diffraction studies, transmission electron microscopy, diffuse reflectance UV/Vis spectroscopy and nitrogen adsorption measurements are used for characterization of the as-prepared β-Bi₂O₃ nanoparticles. The properties of the β-Bi₂O₃ nanoparticles depend on the starting bismuth oxido clusters with regard to particle size and optical band gap. The β-Bi₂O₃ nanoparticles show excellent photocatalytic activity as demonstrated by dye decomposition (rhodamine B, methyl orange, methylene blue and orange G) under visible light.