In this work, we have demonstrated that XPDF data can successfully capture the pre-crystalline stages of Hf MOF formation during in situ solvothermal reactions, including distinguishing between different metal clusters in solution. We have used this technique to explore the response of HfCl4 to different reaction temperatures and solvents, in particular those used to synthesise the hcp UiO-66(Hf) MOF, using a combination of refinement and peak area calculations to track subtle changes in the XPDF between different systems and over the course of in situ reactions. In water the dominant cluster form is the tetramer, but in a mixture of DMF, formic acid and water, it quickly begins to convert to the hexanuclear single cluster form even at room temperature; the formic acid is likely to act as a directing group, but we were not able to identify any intermediates in the transition from tetramer to single cluster. Heating this reaction mixture results in an initial decrease in the proportion of tetramer, followed by the growth of the double cluster alongside a reduction in the amount of single clusters. This provides strong support for a mechanism for double cluster formation directly from pairs of single clusters with μ 2 -OH bridges created between them, rather than forming directly from the tetramer or requiring the single clusters to break down and reform larger clusters. No double clusters were observed at room temperature, suggesting that aggregation to form higher-nuclearity clusters is favoured by higher temperatures and the rate of The insights we have presented here into the behaviour of Hf (and Zr) MOFs during their synthesis advance our understanding of the formation of these important materials. The UiO family of MOFs are particularly promising candidates for real-world applications, for example in energy storage devices, but in order to further unlock their potential, they must be able to be deliberately engineered with application-specific properties. The identity and nature of the mental clusters in the framework greatly influence these properties, and so understanding the mechanisms of their formation is key to designing and engineering new frameworks. This will open up possibilities for the synthesis of as-yet unrealised Zr and Hf MOFs with different cluster species and linkers or even mixed-cluster MOFs.