The formation and growth mechanisms in the hydrothermal synthesis of SnO2 nanoparticles from aqueous solutions of SnCl4 center dot 5H(2)O have been elucidated by means of in situ X-ray total scattering (PDF) measurements. The analysis of the data reveals that when the tin(IV) chloride precursor is dissolved, chloride ions and water coordinate octahedrally to tin(IV), forming aquachlorotin(IV) complexes of the form [SnClx(H2O)(6-x)]((4-x)+) as well as hexaaquatin(IV) complexes [Sn(H2O)(6-y)(OH)(y)]((4-y)+). Upon heating, ellipsoidal SnO2 nanoparticles are formed uniquely from hexaaquatin(IV). The nanoparticle size and morphology (aspect ratio) are dependent on both the reaction temperature and the precursor concentration, and particles as small as similar to 2 nm can be synthesized. Analysis of the growth curves shows that Ostwald ripening only takes place above 200 degrees C, and in general the growth is limited by diffusion of precursor species to the growing particle. The c-parameter in the tetragonal lattice is observed to expand up to 0.5% for particle sizes down to 2-3 nm as compared to the bulk value. SnO2 nanoparticles below 3-4 nm do not form in the bulk rutile structure, but as an orthorhombic structural modification, which previously has only been reported at pressures above 5 GPa. Thus, adjustment of the synthesis temperature and precursor concentration not only allows control over nanoparticle size and morphology but also the structure.