Nanocrystalline thin films of CdS are deposited on glass substrates by chemical bath deposition technique using polyvinyl alcohol (PVA) matrix solution. Crystallite sizes of the nanocrystalline films are determined from broadening of X-ray diffraction lines and are found to vary from 5·4–10·2 nm. The band gap of the nanocrystalline material is determined from the UV spectrograph. The absorption edge is shifted towards the lower wave length side (i.e. blue shift) and are found to be within the range from 2·48–2·8 eV as grain sizes decrease from 10·2–5·4 nm. This is also supported by the spectral response curves. An increase of molarity decreases the grain size which in turn increases the band gap.