In photocatalytic oxidation (PCO) of volatile organic compounds (VOCs), even if using the same UV lamp, reaction rate constant of a photocatalyst denoted as k(I, H)(Lamp) is still affected by operation conditions of light intensity (I) and humidity. It is of great significance to find characteristic parameters of photocatalysts, which excludes the effect of above-mentioned operation conditions and enables direct comparisons of photocatalysts. For this purpose, PCO of formaldehyde in synthetic air in a continuous-flow reactor over transparent films of anatase TiO2 was investigated using three commercially available UV lamps, i.e. UVC, UVA black (UVA1) and UVA white (UVA2). Thereby, inherent rate constant (k(Lamp)) that is only related to the wavelength of UV lamp but independent of light intensity and humidity was found. It suggested that a sequence of k(Lamp) under the three UV lamps, k(UVC) >> k(UVA1) similar to k(UVA2), which, for instance, are 1.6, 0.33 and 0.26 cm(2) s(-1) mW(-1) at low light intensity, respectively. Moreover, humidity impact factor (f(H)) is approximately 3, 0 and -0.4 in three regimes of low, medium and high humidity (LH, MH, and HH), respectively, which manifests positive, zero-sum and negative effects of humidity on the PCO reaction rate. The zero-sum effect of humidity in the MH regime was observed for the first time. Finally, a generalized and quantitative relationship to express k(I, H)(Lamp) as a function of I and humidity was built. (C) 2015 Elsevier B.V. All rights reserved.