Al2O3 atomic layer deposition (ALD) growth with Al(CH3)(3) (trimethylaluminum (TMA)) and H2O as the reactants was examined at the relatively low temperature of 125 degrees C using quartz crystal microbalance (QCM) measurements. The total Al2O3 ALD mass gain per cycle (MGPC) and MGPCs during the individual TMA and H2O reactions were measured versus TMA and H2O exposures. The Al2O3 MGPC increased with increasing H2O and TMA exposures at fixed TMA and H2O exposures, respectively. However, the TMA and H2O reactions were not completely self-limiting. The slower surface reaction kinetics at lower temperature may require very long exposures for the reactions to reach completion. The Al2O3 MGPCs increased quickly versus H2O exposure and slowly reached limiting values that were only weakly dependent on the TMA doses. Small TMA exposures were also sufficient for the Al2O3 MGPCs to reach different limiting values for different H2O doses. The TMA MGPCs increased for higher TMA exposures at all H2O exposures. In contrast, the H2O MGPCs decreased for higher TMA exposures at all H2O exposures. This decrease may occur from more dehydroxylation at larger hydroxyl coverages after the H2O exposures. The hydroxyl coverage after the H2O exposure was dependent only on the H2O exposure. The Al2O3 MGPC was also linearly dependent on the hydroxyl coverage after the H2O dose. Both the observed hydroxyl coverage versus H2O exposure and the Al2O3 ALD growth versus H2O and TMA exposures were fit using modified Langmuir adsorption isotherm expressions where the pressures are replaced with exposures. These results should be useful for understanding low-temperature Al2O3 ALD, which is important for coating organic, polymeric, and biological substrates.