Methanol-assisted supercritical fluid deposition of TiO2 (TiO2-SCFD), which had previously provided uniformly thick films on deep trenches with an aspect ratio (AR) above 30, was kinetically analyzed. To assess the overall reaction kinetics, a novel analytical scheme exploiting micro/macrocavities and flow/closed reactors was developed. Two reaction paths were thereby revealed: surface reaction of the precursor and consecutive reactions starting from homogeneous reactions of the precursor in the fluid (fluid-phase reaction), followed by the surface reaction of the intermediate species. Compared with the original TiO2-SCFD, methanol addition enhanced the surface reaction rate constant and thus growth rate, although the step coverage was slightly deteriorated. Nevertheless, it still had good step coverage on the high-aspect-ratio features (e.g., a step coverage of 85% on a trench with an AR of 50). Methanol addition is an effective way to promote the growth rate insofar as conformal deposition has been ensured on the targeted structure.