Optically transparent and mechanically flexible thin-film transistors (TF-TFTs) composed exclusively of amorphous metal oxide films are fabricated on plastic substrates by combining an amorphous Ta2O5/SiOx bilayer transparent oxide insulator (TOI) gate dielectric with an amorphous zinc indium tin oxide (a-ZITO) transparent oxide semiconductor (TOS) channel and a-ZITO transparent oxide conductor (TOC) electrodes. The bilayer gate dielectric is fabricated by the post-cross-linking of vapor-deposited hexachlorodisiloxane-derived films to form thin SiOx layers (v-SiOx) on amorphous Ta2O5 (a-Ta2O5) films grown by ion-assisted deposition at room temperature. The a-Ta2O5/v-SiOx bilayer TOI dielectric integrates the large capacitance of the high dielectric constant a-Ta2O5 layer with the excellent dielectric/semiconductor interfacial compatibility of the v-SiOx layer in a-ZITO TOS-based TF-TFTs. These all-amorphous-oxide TF-TFTs, having a channel length and width of 100 and 2000 mu m, respectively, perform far better than a-Ta2O5-only devices and exhibit saturation-regime field-effect mobilities of similar to 20 cm(2)/V.s, on-currents >10(-4) A, and current on off ratios >10(5). These TFTs operate at low voltages (similar to 4.0 V) and exhibit good visible-region optical transparency and excellent mechanical flexibility.