Nickel oxide thin films have been widely studied as an optical active layer in electrochromic devices. It is well known for these films that thermal treatment is the key factor which determines the electrochromic effect during potential cycling. Therefore, it is extremely important to perform thermal analysis on the thin films rather than on the corresponding xerogels. In this article, the thermal behaviour of sol-gel derived nickel oxide thin films, prepared from NiSO4 precursor, is compared to that of the corresponding xerogel using dynamic and isothermal TG and dynamic DSC measurements. By regulating the duration of the heat treatment at 270 degrees C, one can control the thermal conversion of the amorphous thin film into nickel oxide. For the xerogel, this process occurs at 300 degrees C. Using the EXAFS method, it was confirmed that the amorphous xerogel is analogous to the thin film sample. Thermal decomposition of the amorphous xerogel was studied in detail by IR and MS techniques. From the weight loss curve of the thin film, the substrate to film mass ratio was determined for different substrates. The effect of heat treatment on these films was determined by measuring their electrochromic properties.