An investigation of electrical properties of indium oxide single crystals has been made. Indium oxide has been found to be an‐type excess semiconductor over a wide temperature range. The electrical conductivity at room temperature is of the order of 10 &OHgr; cm−1and the mobility is approximately 160 cm2V‐sec−1. The temperature dependence of the mobility has been quantitatively interpreted in terms of lattice and ionized impurity scattering. The donor ionization energy has been found to decrease with increasing impurity concentrations. High ``apparent intrinsic'' conductivity with an activation energy of 1.55 eV has been observed at elevated temperatures.