The band gap formation during the first stages of growth of CdS thin films, deposited on glass substrates by chemical bath deposition (CBD) and close spaced sublimation (CSS) techniques was studied. To obtain the first stages of growth, we used deposition times between 5 and 40 min for CBD films, and 4-60 s for CSS films. CBD technique with two chemical bath agitation modalities - the traditional magnetic agitation and ultrasonic vibration - was used for film preparation. Morphological, structural, optical, and stoichiometrical results were obtained on each CdS films group. Higher values of the band gap energy (3.2 eV) were obtained for the thinnest films (2-3 nm) by CBD with ultrasonic vibration; others films, showed similar band gap energy values to those reported for polycrystalline CdS films (2.42 eV). Energy vs. alpha (2) curves, presented strong differences between the two deposition techniques used. The behavior of the optical window effects, the surface roughness, the structural results, and the films thickness related to the deposition techniques are discussed. The CBD technique with ultrasonic vibration produces films with cleaner surfaces, controlled thickness and better optical properties.