The optical absorption spectra of SnO thin films were studied using photothermal deflection spectroscopy, with an emphasis on those around the indirect fundamental gaps. The SnO thin films were fabricated at different substrate temperatures (T-sub), which ranged from 60 to 220 degrees C, by radio-frequency magnetron sputtering. The optical absorption spectra exhibited a steady increase in their intensities in the visible-infrared region and a decrease in the ultraviolet region with increases in T-sub. It is surmised that these trends were due to the promotion of secondary phases, which were caused by disproportionation. Finally, a theoretically derived value of 0.6 eV for the indirect fundamental gap and 2.8 eV for the direct gap were obtained for all the films, irrespective of T-sub, except in the cases in which the p-type conductivity was no longer manifest. (C) 2013 Elsevier B.V. All rights reserved.