The photocatalytic oxidation (PCO) of trace amounts of propane (500 ppm) on nanocrystalline anatase TiO2 has been investigated in situ as a function of temperature (T = 318-473 K), humidity (C(H2O) = 0-4%), and time by means of mass spectrometry and diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). Propane adsorbs associatively on TiO2 at 318 K in dry air, while at 473 K small amounts of thermal dissociation products appear on the surface. In agreement with previous studies, propane is found primarily to be converted to acetone by reactions with photogenerated oxygen radicals. Various successive reaction paths exist, where the branching depends on the temperature and hydroxylation state of the surface. Under dry conditions at 318 K, acetone oxidation is initially kinetically hindered, while, above 400 K, acetone readily decomposes. The thermally assisted reaction channel leads to detrimental bonding of surface species and inhibition of the catalytic activity. It is manifested by a coloration of the sample and suggested to be coupled to surface reduction. Under humidified conditions, there is an optimum of the PCO in C(H2O) and T space, which is estimated to correspond to an equilibrium coverage of one monolayer of H2O (or bilayer). The latter reaction condition also corresponds to sustained high propane conversion and is characterized by rapid establishment of steady state rates. The optimum PCO is discussed in terms of a balance between (i) sustaining enough of a photoactive water monolayer to avoid detrimental bonding of surface species, (ii) allowing reactants to adsorb and access bulk TiO2 photoexcitations, and at the same time (iii) maximizing the thermally assisted decomposition of intermediates.