It is imperative to further explore the in situ formation of Hf and Zr MOFs and their related clusters and precursors, in order to understand their behavior when combined with other species, with different identities and concentrations, which are present under solvothermal conditions. The future of MOF design lies in the rational synthesis of new desired MOF structures, with a range of connectivities and different linkers and subsequently different physical/chemical properties and topologies. In order to do this, through the deliberate design of syntheses to exploit the wide library of possible cluster species, we must improve our understanding of the mechanisms of MOF and MOF-precursor formation. In this work, we bridge the gap between ex situ cluster identification and in situ MOF crystallite growth. We focus our investigation on the UiO-66 family of MOFs, as an archetype of Zr/Hf MOFs. We perform in situ XPDF measurements on reactions of Hfcontaining solutions, under conditions required to form both the hcp UiO-66(Hf) MOF and its molecular cluster precursors. We show that, alongside a careful structure search for plausible cluster models, these XPDF measurements enable us to identify critical cluster intermediates in the materials, including the M6 cluster found in fcu UiO-66(Hf) and the M12 double cluster found in hcp UiO-66(Hf). We also gain insight into the relationship between the cluster formation and the MOF framework coordination. These advances in understanding the stages of growth of UiO family MOFs, including the formation of clusters as precursors, provide routes towards the deliberate and efficient design of MOF syntheses, exploiting the wide library of possible metal cluster species for new and unrealised members of this important class of materials.