Atomic layer deposition (ALD) of TiO2 on SiO2 powder using sequential addition of TiCl4 and H2O vapors has been investigated by infrared spectroscopy. In the first cycle, TiCl4 reacts monofunctionally or bifunctionally with surface silanols forming (Si-O-)nTi-Cl4-n (n = 1, 2) species. Subsequent addition of water vapor leads to the hydrolysis of the (Si-O-)(n)Ti-Cl4-n to form a Ti-O-Ti network, and at the same time, some cleavage of Si-O-Ti bonds occurs, regenerating Si-OH in the process. It is shown that the species formed on the surface in the first TiCl4 dose are temperature dependent. However, after addition of H2O vapor, the amount of TiO2 deposited in the first complete cycle is independent of reaction temperature. In the second and above cycles, the amount of TiO2 deposited as a function of ALD cycles strongly correlates with the amount of water on the surface. This, in turn, led to a temperature dependence of the growth rate of the TiO2 per cycle.