Diamond thin films grown on high resistivity, 100-oriented silicon substrates by the hot filament chemical vapor deposition method have been characterized by four-point probe and Hall effect measurements. The resistivities of both as-grown and chemically etched samples were lower than expected. The Raman spectra showed dramatic changes from two broad bands (one starting at 1250 cm⁻¹ and peaking at 1350 cm⁻¹ and the other starting at 1500 cm⁻¹ and peaking at 1580 cm⁻¹) for the as-grown samples to a sharp peak centered at 1332 cm⁻¹ with a full width at half-maximum of 10.7 cm⁻¹ for the chemically treated sample. Hall measurements yielded carrier concentrations in the temperature range 180–300 K. From a plot of carrier concentration vs. inverse temperature, activation energies of 0.36 and 0.20 eV are obtained for the two chemically treated samples. The resistivity values at room temperature of 100 Ω cm are significantly lower than the values recently observed for other undoped samples (=10³−10⁴ Ω cm). The low resistivity values observed in this work are attributed to the disordered graphitic regions between the diamond crystalline grains.