Reaction mechanisms between titanium isopropoxide and deuterated water in the atomic layer deposition (ALD) of TiO 2 at 150±350 C were studied using a quartz crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS). The temperature had no marked effect on the total amount of the main gaseous by-product (CH 3 ) 2 CHOD. At 150±250 C, about half of the ligands were released in reactions with surface hydroxyl groups during the Ti(OCH(CH 3 ) 2 ) 4 pulse, and the other half during the water pulse. At higher temperatures, Ti(OCH(CH 3 ) 2 ) 4 began to thermally decompose, thus affecting the growth mechanism. The results were compared with earlier CVD and ALD studies.