The morphological evolution and SnO2 crystallization of various gel-derived materials, composed of SiO2-SnO2 in the range from 34 to 5 wt % SnO2, are studied in function of the thermal treatment in the interval 200-800°C. SnO2 phase separation occurring above 400°C affects the density, the apparent porosity and mechanical properties like hardness and Young's modulus. At the lowest SnO2% a continuous density increase is observed on heating at variance with high tin dioxide contents for which the apparent density is almost constant. The results are discussed on the basis of SnO2 crystallization, which occurs by two different mechanisms, evidenciated by X-ray analyses. The data will be compared with those of pure SiO2.