This paper describes the correlation between the nature of iron salt in the spray solution and the microstructure, phase and chemical composition of SnO2-Fe2O3 thin films, deposited by spray technique. The dependence between humidity sensitive parameters and the chemical and microstructural characteristics have also been studied. It was found that the films exhibit an homogeneous fine grain structure. Part of the introduced iron precipitates in large crystallites on the surface. The fine grain film structure has been obtained using both Fe2(C2O4)3 or Fe(NH)4(SO4)2·12H2O as iron precursors. When iron oxalate is used, finer microstructure is obtained (10-20 nm smaller grain sizes). The concentration of doping element in the homogeneous layer is independent of the type of the used iron salt. In the large crystallites, however, when an organic precursor is used the iron content is twice as low than in the case of the iron sulphate. The quantity and type of iron salt used strongly affect the humidity, sensitivity and electrical resistance of the layers. © 1996 Chapman & Hall.