Indium oxide and tin-doped indium oxide thin films have been deposited by reactive thermal deposition technique on semi-insulating (n=1 X 10(8) cm(-3)) indium phosphide substrates using elemental indium and indium-tin alloy sources. The depositions were carried out for various substrate temperatures ranging from 50 degrees C to 310 degrees C. Phosphorous out-diffusion from the indium phosphide substrate was identified for a lower heating rate (3 degrees C min(-1)) above a 250 degrees C substrate temperature (T-s). The heating rate was changed from 3 degrees C min(-1) to 7 degrees C min(-1) and good quality indium oxide and indium tin oxide thin films were deposited on InP substrates. The deposition was also carried out on polycrystalline InP substrates to understand the orientation and grain boundary effects on deposited films. Optical microscope observations showed distinguishable separation in the surface structure of the deposited thin films on the polycrystalline substrate along the grain boundary region. X-ray diffraction studies revealed the polycrystalline structure for as-deposited IO and ITO films on InP single crystal substrates. Low resistivity 9.33 X 10(-4) Ohm cm was obtained for the tin-doped indium oxide films. The results obtained are discussed in detail.