The photocatalytic decomposition efficiency of gas-phase acetaldehyde was studied using a titanium dioxide thin film under weak UV illumination. Acetic acid and carbon dioxide were detected as the main reaction products. It was found that the apparent quantum yields (QY) for acetaldehyde degradation are determined by the normalized absorbed photon number (I-norm /s(-1)), a parameter that is defined as the ratio of the number of absorbed photons to the number of adsorbed acetaldehyde molecules. This result is similar to that for 2-propanol degradation reported previously (Ohko, Y.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. A 1997, 101, 8057). However, although the QY values for 2-propanol degradation reached a constant value (ca. 28%) for very low relative light intensity (in the I-norm region less than 10(-4) s(-1)), those for acetaldehyde degradation continued to increase with decreasing I-norm and reached 180% for an initial concentration of 1000 ppmv, at an I-norm value of 3 x 10(-5) s(-1). This discrepancy is due to the existence of radical chain reactions for the latter reaction. Compared to the maximum QY yield for 2-propanol decomposition (28%), which involves no chain-type reactions, the maximum QY for acetaldehyde conversion to acetic acid (similar to 150%) implies a radical chain-type process with a chain length of approximately five.