Sintering of supported nanocatalysts often leads to the loss of the catalytic activity and selectivity. This paper reports on synchrotron X-ray studies of the thermal stability of supported platinum nanoparticles produced by cluster deposition on the naturally oxidized surface of a silicon wafer (SiO2/Si(111)). The temperature region of aggregation was determined by gradually heating the samples up to above 400 degreesC, and recording two-dimensional in situ X-ray scattering images during the heat treatment. The data analysis reveals an unexpectedly high stability of the supported particles, which preserve their original size up to about 320 degreesC, at which an abrupt onset of the agglomeration takes place.