Modified tin dioxide nanoparticles of enhanced thermal stability were prepared by sol–gel technique. The main point of the procedure is phosphoric acid treatment of the tin oxide hydrate (SnO 2· nH 2O). During the procedure, the molar ratio of P:Sn was varied in the range of 0.01–3.4. The structure and morphology of the modified SnO 2 particles (P-SnO 2) and that of two reference samples (SnO 2 and SnHPO 4/SnP 2O 7) were studied with various methods. X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) measurements reveal that there are various tin phosphate compounds on the surface of SnO 2 particles, improving the thermal stability of the original particles. The phosphate-containing shell formed on tin oxide hydrate inhibits sintering of the particles during calcination. The amorphous untreated SnO 2 particles undergo crystallization at around 300 °C (relatively sharp X-ray (1 1 0), (1 0 1) and (2 1 1) reflection peaks appear), whereas P-SnO 2 particles remain amorphous up to 550 °C. At 1000 °C, the untreated SnO 2 continue to crystallize into bulk material, while the crystal growth of P-SnO 2 is inhibited: the average SnO 2 crystallite size at P:Sn = 0.01 molar ratio remains 27.7 nm. Increasing phosphoric acid concentrations resulted in decreasing sintering effect during calcination. The band gap energy of the investigated particles was determined from UV–vis–DR measurements.