CuO films of 51 mu m thickness have been fabricated from nanocrystalline powder, which has been synthesized by a sol-gel auto-combustion method at different pH values of the precursor solution. Studies reveal that the pH value of the precursor solution strongly affects the decomposition rate of the metal-citrate complex formed by precursors (cupric nitrate and citric acid). Structural characterization of the powder samples shows a considerable change in agglomeration behavior, crystallite size and strain with variation in pH value of the precursor solution. Studies show that high pH reaction conditions results in the production of highly porous CuO nanoparticles with lowest crystallite size of 27 nm. Thick films of the synthesized material show an extremely high response of 0.941 to few parts per million level of ammonia at room temperature as well as possesses good stability for a long period of time. The adsorption of ammonia on the sensor surface obeys Elovich equation and the reaction kinetics followed is of first order. The lowest potential barrier of 0.50 M Omega and highest rate constant of 0.0136 s(-1) have been found for ammonia adsorption on the sensor surface in case of film fabricated from CuO powder synthesized at high pH value of precursor.