BACKGROUND In treatment of air using UV-irradiated titanium dioxide (TiO2), volatile organic compound (VOC) concentrations mostly reach zero or decrease to a miniscule amount. However, when acetaldehyde (AA) is treated by photocatalysts immobilized on silica or activated carbon (AC) particles, which have a high ability to adsorb VOCs, the AA concentration decreases rapidly initially, but decreases very slowly after AA has been decomposed to a certain low concentration. To overcome this problem, the performances of photocatalysts immobilized on different supporting materials were investigated. RESULTS Computer simulations clarify that the slow decrease in the AA concentration in the final stage is due to the desorption of adsorbed AA, taking a much longer time to completely decompose AA. The simulations also suggest two strategies to overcome the problem: using a support with a smooth surface and low adsorption ability, and increasing the photocatalytic activity. The first strategy was tested by immobilizing TiO2 on glass tubes, PET film sheets and aluminum foil sheets with low adsorption abilities. The experimental results indicate that the AA concentrations for these supports decrease at lower rates than for supports with high adsorption ability, but decrease steadily toward zero. The second strategy was tested by increasing the amount of immobilized-photocatalyst particles with high adsorption abilities, resulting in a shorter time for the AA concentration to reach zero. CONCLUSION For completely decomposing AA by an immobilized photocatalyst in a short period of time, the use of supporting particles with a high adsorption ability should be avoided. (c) 2020 Society of Chemical Industry