The hydrothermal transformation of the heteroatom polyoxovanadate compound {Ni(en)3}3[V15Sb6O42(H2O)]·≈15H2O (en = ethylenediamine) in water into [{Ni(en)2}2V14Sb8O42]·5.5H2O was investigated by in situ XRD experiments at different temperatures. First, the precursor undergoes very fast amorphization or dissolution, and an induction period is observed that depends on the reaction temperature. Crystal growth of the product is completed within 3 h, while higher temperatures lead to accelerated reaction progress. Evaluation of the kinetics showed that heterogeneous nucleation is the rate-limiting step of the reaction. The activation energies for nucleation and crystal growth are very low compared to data reported in the literature for several other chemical systems.