Solution-processed high-κ gate dielectrics, such as Al 2 O 3 and HfO 2 , show great potential for use in low-cost electronics. However, high-temperature treatments are generally needed to cure the film, limiting the use in flexible electronics. Here we use anodization, a simple, low-cost, solution-based method, to form thin, conformal, high quality HfO x layers at room temperature. Several anodization voltages were studied, and the relative quantity of Hf−O bonding was found to increase by 10% when increasing anodization voltage from 2 to 3 V, while maintaining almost the same when further increasing the anodization voltage. Conduction mechanisms were analyzed, showing that Schottky emission dominates even for the thinnest oxides. HfO x layers formed by anodization voltages of 2, 3, and 4 V show high capacitance density of 1290, 1160, and 1060 nF/cm 2 , which are equivalent to 2.68, 2.97, and 3.26 nm of thermally grown SiO 2 . Using such high capacitance density insulators, we fabricated InGaZnO (IGZO) thin-film transistors (TFTs) at room temperature, showing a low operating voltage of 0.5 V, one of the lowest for oxide−semiconductor-based TFTs. TFTs gated with HfO x anodized at the optimal voltage, 3 V, show a high current on/off ratio of >10 5 , a mobility approaching 10 cm 2 /(V s), and a subthreshold swing as low as 69 mV/dec, close to the theoretical limit at 300 K. The interface trap density was found to be highly related to the quality of the HfO x film, showing a 50% decrease with the increase of anodization voltage from 2 to 3 V. The devices show negligible change after storage in air for 6 months and favorable stability under both positive and negative gate bias stress. Finally, we demonstrate TFTs on plastic substrates, maintaining high quality performance even under a bending radius of 9 mm. As a result, such devices have potential applications in low-cost, low-voltageoperation, low-power electronics.