The purest indium arsenide so far reported has a carrier concentration of about 1.3.1016 electrons/cm3 at room temperature. This is several times the intrinsic value and it has been suggested that the limiting impurity may be sulfur originating in the arsenic component. Arsenic has been prepared by the thermal decomposition of highly purified arsine gas and has been combined with samples of indium purified by a variety of processes. Mass spectrographic analysis of the resultant ingots showed in all cases a sulfur content less than 0.05 ppm. The purest sample had a Hall coefficient of 795 cm3/coulomb at 77°K, corresponding to an electron concentration of 8.1015/cm3 while the Hall mobility was 75,700 cm2/v/sec. Analytical techniques failed to detect the prime source of the free electron concentration, but it is concluded that sulfur is not the dominant impurity in these samples.