As elucidated previously, classical nucleation theory does not fully describe nucleation of nanostructures because it does not cover so-called non-classical nucleation pathways like e.g. formation via cluster clouds . However, it provides reliable information on the minimum size at which a nanostructure can be regarded as stable. This is quantified by the critical radius rcrit, which is reliably accessible during nanoparticle dissolution or etching . It is defined by the Gibbs free surface energy γ, the atomic volume Ω, and the difference in the chemical potential between the initial and the final phase under equilibrium conditions Δµ. 𝑟 𝑐𝑟𝑖𝑡 = 2𝛾𝛺 𝛥𝜇 (2) At 1 mM, Figure (a) suggests no significant difference in chemical potential, as the steadystate mol fraction of Au 0 is expected to amount to 1 here (Figure ). Consequently, the transitions observed are expected to be reversible. Hence, the state can be interpreted as a dynamic equilibrium. This is in line with the experimental findings in Figure showing reversible growth and dissolution. Furthermore, at 1 mM the average value of rcrit is in the same range as values obtained for Pt-nanoparticles in 0.228 M FeCl3 aqueous solution. The reversible process is still driven by the electron beam, because it defines the dynamic equilibrium condition itself (i.e. by changing the solution chemistry). However, this suggests that such dynamic equilibria could be well-suited for analyzing material-specific parameters.