In this paper, we have shown that in water, HfCl4 forms the tetramer cluster [Hf4(OH)8•16H2O] 8+ , surrounded by a structured coordination shell of water, as expected at room temperature. With a change of solvent system to DMF : formic acid : water, we obtained a mixture of the tetramer and the single cluster (with a core of [Hf6(μ 3 -O)4(μ 3 -OH)4]), with less structured solvent surrounding the clusters. This suggests that (alongside the DMF solvent) the acid is a critical species in causing the formation of the single cluster. However, in this room-temperature experiment, both the tetramer•H2O and the single cluster coexisted in solution, even though both water and formic acid were in large excess (modulator : metal molar ratio of 183:1). This contrasts with the analysis in a previous XPDF investigation (of Zr species rather than Hf), which suggested that solutions of ZrCl4 in either DMF/HCl (37 wt% in water) solvent (1:10.3:41.5 molar ratio metal : acid : water) or in DMF/H2O solvent without added acid (1:28.8 molar ratio metal : water) produced predominantly the single cluster. Although the hydrolysis reactions necessary for cluster interconversion are known to be kinetically hindered, that study employed neither heating nor ageing to overcome these kinetic limitations, so it is surprising that a mixture of single cluster and tetramer was not observed, especially given the lower concentrations of acid and water relative to metal in that study. Our observation in this work of the importance of acid to the formation of the single cluster is corroborated by an EXAFS study, which revealed that the addition of acetic acid to the tetramer in aqueous solution triggers a rearrangement via a (too short-lived to be identifiable) intermediate to form the single cluster. In the EXAFS study, a mixture of the two clusters (as we observed here) was only seen with modulator : metal ratios below 10:1, which is a far smaller ratio than in our work, despite acetic acid being less reactive than the formic acid modulator we use. This difference is likely because the reaction solutions in the EXAFS study were aged for weeks prior to measurement in order to obtain equilibrium, which will greatly affect the degree of cluster hydrolysis and conversion.