Electrochromic thin films were prepared from nickel chloride NiCl2 precursor by the sol-gel method. To obtain reversibility of the colouring/bleaching process and maximal change in transmittance during potential switching, thin films should be thermally treated to an optimal degree. From dynamic TG measurements of the sol-gel prepared thin films, deposited on a substrate, a temperature was determined at which makes it essential to prepare several films thermally treated to a different extent by changing the duration of the treatment at this temperature. Electrochromic response of such prepared films was additionally tested by spectroelectrochemical measurements. From the comparison of dynamic TG measurement of thin film and the corresponding xerogel it is evident that xerogel starts to decompose at considerably higher temperature. Therefore the results obtained for xerogel samples could not be used for optimization of the thermal treatment of thin films. The described optimization was done for the originally prepared films and also-for films in which the content of the counter ions (chloride, lithium) was lower. From the results of spectroelectrochemical measurements, optimized thermal treatment for both thin films is reported together with the change in transmittance between the bleached and the coloured state (at lambda = 480 nm) and their coloration efficiency. TEM of cross-sections revealed the structure and thicknesses of both films.