Zinc sulfide (ZnS) films were deposited by chemical bath deposition (CBD) method using zinc sulphate (ZnSO4) and thioacetamide (TA) as starting materials. For doping the ZnS films, MnCl2 was added to the solution. The changes in optical constants were studied as a function of annealing temperature. By annealing, the energy gap (E-g) values were seen to be reduced in both doped and undoped films. The Mn doped films were seen more resistant to the heat treatment than undoped films. The optical absorption coefficient changes were observed in the films.