Indium tin oxide layers were deposited by reactive thermal evaporation of an In:Sn alloy in an oxygen ambient. The films were evaporated from tungsten (W) and tantalum (Ta) crucibles using a large range of tin concentrations. The influence of the crucible material and the tin concentration on the optoelectrical and electrical properties was studied. A minimum film resistivity of (2.8+/-0.5) x 10(-4) Omega cm and a maximum optical transmittance of 91% were obtained for the films evaporated from a tantalum crucible containing 70% tin [deposition rate of (0.6+/-0.3) Angstrom/s and oxygen partial pressure of 1 x 10(-3) mbar]. The atomic tin concentration in these films is (0.4+/-0.06)% at maximum. The resistivity decreases monotonically with increasing tin concentration in the source alloy up to 70 at. %. The films deposited from a tungsten crucible showed similar low resistivity, (3.4+/-0.2) x 10(-4) Omega cm, using only 10 at. % Sn in the source alloy. It was found that this low resistivity obtained by using a W crucible is not the result of tin doping (<0.05 at. %), but is caused by the incorporation of tungsten impurities (similar to 0.8 at. %) into the film. The presence of W in these films can be explained by oxidation of the tungsten crucible. The resulting WO3 has, unlike W, a much higher vapor pressure than Sn, which implies a higher evaporation rate. No traces of Ta were found in films evaporated from a Ta crucible.