The hydrolysis of titanium tetrachloride (TiCl4) to produce titanium dioxide (TiO2) nanoparticles has been studied to provide insight into the mechanism for forming these nanoparticles. We provide calculations of the potential energy surfaces, the thermochemistry of the intermediates, and the reaction paths for the initial steps in the hydrolysis of TiCl4. We assess the role of the titanium oxychlorides (Ti x O y Cl z ; x = 2−4, y = 1, 3−6, and z = 2, 4, 6) and their viable reaction paths. Using transition-state theory and RRKM theory, we predicted rate constants including the effect of tunneling. Heats of formation at 0 and 298 K are predicted for TiCl4, TiCl3OH, TiOCl2, TiOClOH, TiCl2(OH)2, TiCl(OH)3, Ti(OH)4, and TiO2 using the CCSD(T) method with correlation consistent basis sets extrapolated to the complete basis set limit and compared with the available experimental data. Clustering energies and heats of formation are calculated for neutral clusters. The calculated heats of formation were used to study condensation reactions that eliminate HCl or H2O. The reaction energy is substantially endothermic if more than two HCl molecules are eliminated. The results show that the mechanisms leading to formation of TiO2 nanoparticles and larger ones are complicated and will have a strong dependence on the experimental conditions.