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Over the past decade, there have been many reports on solution-processed oxide thin-film transistors (TFTs) with high mobility (even >100 cm² V⁻¹s⁻¹). However, the capacitance uncertainty of the solution-processed oxide gate dielectrics leads to serious overestimation of the mobility. Here, solution-processed AlO_x dielectrics are investigated systematically, and the effect of mobile ions on the frequency-dependent capacitance of the solution-processed AlO_x dielectrics is also studied. It was found that the capacitance of the AlO_x depends on the frequency seriously when the annealing temperature is lower than 300 °C, and the water treatment causes more seriously frequency-dependent capacitance. The strong frequency-dependent capacitance of the AlO_x annealed at 250 or 300 °C is attributed to relaxation polarization of the weakly bound ions in the incompletely decomposed AlO_x films. The water treatment introduces a large number of protons (H⁺) that would migrate to the ITO/AlO_x interface under a certain electric field and form an electric double layer (EDL) that has ultrahigh capacitance at low frequency.