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 2 O). 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 2 O 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 (110), (101) and (211) 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 27.7nm. 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.