The role of moisture (H2O) in atomic-layer-deposited (ALD) HfO2 and HfSiOx dielectrics on equivalent oxide thickness (EOT) scaling is investigated in gate-first devices with physical-vapor-deposited TaN gates. By reducing the amount of H2O present in the gate stack, the EOT increase during high-temperature (1030 degrees C) spike annealing is reduced from 0.5 to 0.25 nm. This was achieved by a premetallization degas at 350-550 degrees C for 3 min. The origin of this improvement lies in the elimination of H2O-related reoxidation of the interfacial SiO2. By means of temperature-programmed desorption measurements, it is found that the majority of the H2O is desorbed between 200 and 600 degrees C with activation energies between similar to 0.3 and similar to 0.5 eV. Furthermore, the H2O concentration is dependent on the Hf concentration and nitridation. Non-nitrided HfSiOx with [Hf]=40% adsorbs twice as much H2O as HfO2. Correspondingly, a significantly larger EOT increase is observed for HfSiOx with [Hf]=40% as compared to HfO2. In HfSiON, the desorbed H2O concentration is independent of [Hf]. Finally, a strong metal dependency on the EOT regrowth is observed. With ALD TiN, the EOT increase after thermal treatment is found to be independent of the degas condition at similar to 0.2 nm.