Abstract A mixture of SnCl2 and tris(acetylacetonato)manganese(III) complex was thermally co-decomposed to synthesize Mn-doped SnO2 powders. Two preparation routes were conducted: in the first route, the samples were annealed in air and in the second route, the samples were further annealed in high vacuum. The elemental and crystalline structures of the prepared powders were studied by the X-ray fluorescence (XRF) and X-ray diffraction (XRD) methods. It was confirmed that all the incorporated Mn ions doped in SnO2 forming Sn1−xMnxO2 substitutional solid solution (x=0.000, 0.014, 0.024, 0.068). The samples were subjected to an extensive optical characterisation with FTIR and optical absorption methods. The doping processes as well as the vacuum-annealing effect were studied by FTIR method. The optical absorption spectroscopy was used to observe the dependence of the absorption onset on the Mn-doping level. Annealing in vacuum was proved to create more oxygen vacancies.