Highly transparent crystalline manganese sulfide (gamma-MnS) thin films were prepared on glass substrate by chemical bath deposition (CBD) method at room temperature (27 degrees C). The refractive index n(lambda), extinction coefficient k(lambda) and film thickness t are evaluated from spectrophotometric transmittance spectrum by using envelope method. The optical band gap of the film was estimated to be 3.88 eV. XRD measurements show that the MnS films are crystallized in the wurtzite phase and present a preferential orientation along the c-axis. The effect of trisodium citrate on crystalline films has been determined. The grain size of the film is estimated to be 29.8 nm using by the Scherrer's formula. 2006 Elsevier BN. All rights reserved.