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Vanadium oxide (VO x ) thin films were deposited at various substrate temperatures ( T s ) by spray pyrolysis technique using 0.05 M vanadyl acetylacetonate precursor. V 4 O 9 films are formed at T s = 300 °C, while mixed V 2 O 5 phases are formed at higher T s (400 and 500 °C). Annealing in forming gas of V 4 O 9 films shows the formation of higher content of thermochromic VO 2 phase than V 2 O 5 films. V 4 O 9 films show little higher electric resistivity ( ρ ), higher temperature coefficient of resistance ( TCR ), and higher thermal carrier activation energy ( E a ) than V 2 O 5 films. Annealed VO x films show a 2–3 order of magnitude change in ρ , optical transmission switch of 19–39%, and higher E a than the as deposited films. Annealed films deposited at T s = 500 °C presents a high TCR of −4.6%K −1 . Optical absorption, electronic transitions, and energy gaps of the formed VO x phases have been discussed in relation to its electronic band structure. [ABSTRACT FROM AUTHOR]