Oxide semiconductor electronics may enable new applications including large-area, flexible, integrated systems. ZnO thin film transistors have been reported with field-effect mobility > 100 cm2/V·s, on-current density > 700 mA/mm, and microwave operation (fT > 2 GHz, fmax > 7 GHz) for ZnO deposited by pulsed laser deposition at 400°C. Other oxide semiconductors, including amorphous and crystalline mixtures of I2O3, Ga2O3, ZnO, have also been widely studied, and high mobility (> 30 cm2/V·s) thin film transistors and circuits with propagation delays < 1 ns/stage have been reported.[2,3] However, most of these high performance demonstrations were done on single crystal semiconductor substrates with high thermal conductivity. Here we find that self-heating and not drain-induced barrier lowering as previously reported  is the physical mechanism responsible for the output conductance (gd = dIDS/dVDS) observed in a range of oxide thin film transistors. In particular we find that self-heating is a significant limiting factor for the performance of oxide devices and circuits on low-cost, low-thermal conductivity substrates such as glass and plastic.