Pure metastable beta-MnS thin films have been deposited by chemical bath deposition (CBD) method and subsequently annealed in a Na(2)S solution (denoted as hydrothermal annealing). The effects of preparative parameters and hydrothermal annealing on structure, morphology and optical property of the films have been investigated. Experimental results indicate that the crystalline beta-MnS thin films can be prepared at a low concentration of Mn and S ions without using any organic chelator. The as-deposited beta-MnS can be transformed to gamma-phase MnS after the hydrothermal treatment in an autoclave at 200 degrees C for 1 h. The estimated E(g) values are in the range of 3.15-3.18 eV. Published by Elsevier Ltd.