[en] Here we provide evidence that the formation of PuO${}_2$ nanoparticles from oxidized Pu${}^{VI}$ under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH${}_4$PuO${}_2$CO${}_3$, which is stable over a period of several months. For the first time, state-of-the-art experiments at Pu M${}_4$ and at L${}_3$ absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase. (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)