The photo-oxidation of propane in dry and humid synthetic air was studied on anatase and rutile nanoparticles by operando diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy and mass spectrometry (MS). Analysis of the propane removal and CO₂ production rate shows that the carbon mass balance (CMB) is CMB < 1 under all reaction conditions up to 1 h operation, and that it is significantly higher on rutile than anatase in humid reaction gas. Corresponding analysis of DRIFT spectra acquired simultaneously as the MS measurements shows that the CMB data scales with the concentration of surface species, i.e. the deviations from unity in the reactant-product mass balance (1-CMB) is quantitatively accounted for by a corresponding increase of surface species measured by DRIFT. The identity of surface species and rate determining reactions steps is deduced by fitting the mode-resolved vibrational IR bands to a kinetic model that accounts for observed temporal evolution of absorption bands. Photo-oxidation of acetone and formate is found to be rate determining on anatase and rutile, respectively. The importance of water is to hinder these surface species to bind to Ti cation surface sites; in particular the concentration of bridging-bidentate formate is reduced on rutile, which under dry conditions exhibits the lowest photo-degradation rate.