In this work, we demonstrate the influence of the oxidative treatment of carbon nanotubes (CNTs) on the morphological, optical, and photoelectrochemical properties of CNTs-TiO2 photoanodes. CNTs were synthesized by a chemical vapor deposition method (CVD) and subsequently oxidized in a mixture of HNO3/H2SO4 to introduce nitrogen- and oxygen-containing functional groups. The oxidation time was varied (45, 90, and 180 min) to obtain CNTs with different oxidation levels (CNTs-ox). Pristine and oxidized CNTs were deposited on ITO and used as substrates to fabricate ITO/CNTs/TiO2 photoelectrodes by depositing a TiO2 thin layer using a CVD procedure. The extent of oxidation of the CNTs was found to directly influence the homogeneity and morphology of the TiO2 overlayer, its physical stability, and the optical and photoelectrochemical properties of the photoelectrodes. An optimum oxidative treatment was achieved for 90 min, leading to a CNTs-ox/TiO2 electrode with the best performance. Compared to shorter and longer times, this sample showed the smallest bandgap and the best homogeneity of TiO2 growth on the CNTs' surface. Optimal photoelectrochemical performance was also found for this sample (160 μA/cm2 at 0 V vs. Ag/AgCl) in KCl (0.1 M) electrolyte (100 mW/cm2, Xenon arc lamp). Graphical Abstract: [Figure not available: see fulltext.].