This paper reports on the comparative investigation of structural and optical properties of nano thin films of ZnO and Sr-doped ZnO (SZO) onto glass substrates synthesized by a two-step chemical bath deposition (CBD) technique. The mode of crystallization, structural properties, and morphologies have been investigated. The films are polycrystalline in nature with hexagonal phase having (002) preferential orientation. The typical crystallite size is also estimated and found to be around 30-80 nm. The shifts in optical band gap of the SZO films are estimated to be similar to 3.25-3.27 eV with respect to the ZnO film and the refractive index is 2.35. The room temperature resistivity is of the order of similar to 2,000 Omega cm. Thermoemf measurements show that films are of n-type. The sensitivity of the films was studied as a function of their temperature 275-575 K for a fixed ethanol concentration (400 ppm). The films have been tested for cross sensitivity for different gases and it has been confirmed that these are highly sensitive and selective for ethanol vapors around 200 degrees C in air atmosphere.