Atomic layer deposition of TiO2 films from alternate pulses of TiI4 and H2O-H2O2 vapors was studied with a quartz crystal microbalance in realtime. The film formation mechanism did not depend remarkably on the time parameters of growth cycles or precursor doses but was dominantly determined by the growth temperature. The reaction of H2O-H2Os with the TiIx-terminated surface was a self-limited process. The adsorption of Tit was not entirely saturative but proceeded via partial decomposition of TiI4, as the adsorbed mass increased continuously during the TiI4 pulse. Changes in the growth mechanism and an increasing contribution of precursor decomposition were observed at temperatures between 200 and 300 degrees C.