The photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW-45 mu W . cm(-2). Under such low-intensity UV illumination, the value of the quantum yield (QY) increased gradually with decreasing number of absorbed photons and finally saturated (28%) for a number of absorbed photons less than 4 x 10(11) quanta.cm(-2).s(-1) for an initial 2-propanol concentration of 1000 ppmv. Thus, purely light-limited conditions were reached. For lower initial concentrations, the QY values decreased, but the same maximum QY value as that for 1000 ppmv was also approached with decreasing light intensity. We discuss these results in terms of the normalized absorbed photon number (I-norm/s(-1)), a parameter that we have defined as the ratio of the number of absorbed photons ([photon](ab)) to the number of adsorbed 2-propanol molecules ([M](ad)). When all of the experimental QY values were plotted as a function of I-norm, all of the points appeared on a single line for a wide range of initial 2-propanol concentrations. On the basis of these results, we conclude that either (OH)-O-. radicals or 2-propanol molecules must be able to diffuse at least ca. II nm on the titanium dioxide surface in order to react with each other. We also conclude that the maximum QY value of 28% represents the intrinsic charge-separation efficiency for this photocatalyst.