Zinc-manganese oxide thin films prepared on glass and Si substrates have been annealed at different conditions in order to create different crystalline structures and agitate a solid-state reaction. The films were characterised by energy dispersion X-ray fluorescence (EDXRF), X-ray diffraction (XRD), and UV-vis absorption spectroscopy. The EDXRF spectrum was used to determine the weight fraction ratio of Mn/Zn in the prepared samples. The XRD method was used to determine the crystalline structure of the phases created in the sample during the pre-annealing processes at different temperatures (400, 600, 800, and 1000 degrees C) in air and in vacuum. The spectral transmittance and reflectance of amorphous and polycrystalline Zn-Mn oxide samples grown on glass substrates in the fundamental absorption region of the spectrum were studied. The spectral refractive index as well as extinction coefficient was determined. The optical bandgap and its variation under crystallisation were determined. The spectral real and imaginary parts of the optical dielectric constant were calculated and explained by a molecular energy diagram. (C) 2007 Published by Elsevier B.V.