A complex kinetics of the photocatalytic decomposition of gaseous ethylene on nanoparticulate Fe(III) doped anatase TiO2 was analyzed in a continuous-flow fixed-bed reactor. The analysis evidenced the production of non-volatile intermediate reaction products, which cover the photocatalyst surface and delay the time of attaining the stationary reactor performance. As a result, the reaction yield slowly increases during the long-lasted (similar to 3 h) intermediate stage. Based on Raman, FTIR and thermal post-treatment analyses, we assigned the major non-volatile reaction product to polymeric oxidized hydrocarbons: furan and furfural oligomers. A theoretical model is proposed explaining the observed kinetics. (C) 2019 Elsevier Inc. All rights reserved.