The effects of organic impurities adsorbed or incorporated into semiconductors structure on the photocatalytic products are noteworthy. For this purpose, the as-prepared Ag, Au and Pd-modified TiO2 samples were exposed to UV vis irradiation in various gas atmospheres (CO2, N-2, 600 ppm CO2 in N-2, and (CO2)-C-13) in order to clarify the route of CH4 formation in the process of photocatalytic CO2 reduction. In the presented research it was shown that in N-2 atmosphere the high concentration of methane was formed. For the removal of the organic adsorbates from photocatalysts the elongated calcination process was applied, however, it occurred insufficient for carbonless TiO2 preparation. The highest amount of methane was formed in the presence of the 0.5Ag-TiO2 sample which contained the highest amount of carbon. Experimental data, including isotope labelling results, confirmed that the obtained methane was formed from the organic impurities incorporated into TiO2 structure and not from the (CO2)-C-13. (C) 2014 Elsevier B.V. All rights reserved.