The effect of heat treatments between 400°C and 500°C on electrical contacts of In, Ga, Pb, and AU to n- or p-GaAs, and Ni to n-GaAs have been studied using current-voltage (I-V), Auger electron spectroscopy and scanning electron microscopy. Ohmic contacts were formed between In and GaAs upon heating due to formation of an InGaAs surface layer with low interfacial transport barriers. The contacts between Ga or Pb and n- and p-GaAs remained rectifying due to a lack of interfacial reaction. Films of Au reacted with both n-and p-type GaAs resulting in rectangular "pits". This led to ohmic contacts only on n-type substrates due to segregation of the Si dopants to the surface of the decomposed GaAs "pits". Dopant segregation did not occur when Au reacted with p-GaAs or Ni reacted with n-GaAs, thus these contacts remained rectifying at lower doping densities (4 x 1017 cm-3).