Microstructure evolution from colloidal solution of SnCl4•5H2O to SnO2 powder and thin film was examined using x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) in addition to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Samples of different stages including sol, colloidal precipitates, (wet) gel, xerogel, powder and thin films were studied. The DSC results indicate that the colloidal precipitates agglomerate more readily as the pH value of the solution decreases. SnO2 crystallization in the precipitates is observed by TEM, which indicates a much earlier commencement of crystallization than that seen in the XRD patterns. XRD results also indicate an enhancement of crystallization of SnO2 and formation of NH4Cl•NH2OH crystalline phase in the gel by a high concentration of Cl(superscript -) ions. SnO2 crystallization in the coated films does not take place as readily as that in the dry powder due to the suppression of clustering by the spinning action.