<p>A systematic quantum chemical study on the spatial structure and energy characteristics of the cluster models for nanoparticles of hydrated forms of tin dioxide (SnO₂)_x⋅yH₂O has been carried out by the second-order Möller-Plesset perturbation theory with the SBKJC valence-only basis set expanded by polarization d and p functions and with respective effective core potential, and the formation mechanisms of the simplest nanostructures from the initial forms of tin hydroxide have been elucidated. It has been shown that the formation of the dimer (SnO₂)₂⋅4H₂O due to association of two Sn(OH)₄ molecules is energetically most advantageous, and a possible mechanism of its formation is proposed. Various forms for trimeric and tetrameric structures of tin hydroxide have been considered. The most energetically profitable isomers are characterized by double oxygen bridges. Besides, some of them are additionally strengthened by intermolecular hydrogen bonds. Further transformations of the nanoparticles lead to an increase in their size, dehydration, and the formation of denser structures that have crystallinity features inherent in solid-phase SnO₂.</p>