TiO 2 thin films have been grown on amorphous soda lime glass and polycrystalline silicon substrates from TiI 4 and H 2 O 2 by atomic layer deposition (ALD) in the temperature range 250±490 C. The film growth rate and refractive index increased linearly with growth temperature up to 300 C. Between 300 C and 400 C, the film growth rate tended to stabilize. Above 400 C, there was a further rapid increase in the growth rate, together with a corresponding increase in the thickness profile. The refractive index reached 2.70±2.75 in the films grown above 300 C. The films contained low amounts of residual hydrogen and were virtually iodine-free. When deposited below 300 C and above 325 C, the films contained weakly crystallized, but still distinct, anatase and rutile phases, respectively. Reflective high-energy electron diffraction (RHEED) studies revealed that the uppermost layers of the films grown on silicon at 275 C, 325 C, and 425 C contained anatase phase, regardless of deposition temperature. In the temperature range 300±325 C, a transition region was established where either the films became less ordered, or they contained rather strongly ordered, but unstable, suboxide phases. The suboxide phase, when present, was transformed into a mixture of anatase and rutile when stored in air at room temperature.