The effects of rapid thermal annealing on the electrical properties of ITO/n-GaAs are investigated. For as-deposited contacts the current is dominated by recombination at low bias and by thermionic emission at high bias. Annealing at 500 degrees C for 20 s causes the current to be dominated by thermionic emission for all biases. Annealing at 600 and 700 degrees C degrades the contact properties. At 700 degrees C annealing temperature the current is dominated by tunneling. Removal of sputter generated defects at low annealing temperature, diffusion of indium and tin atoms into GaAs and formation of an n(+)LnAs-InxGa1-xAs layer at higher annealing temperature are proposed as mechanisms to explain the observations.