Temperature dependent Hall effect measurements were performed for the undoped CdO films with carrier concentrations (n) ranging from 2.4 x 10(19) to 2.0 x 10(20) cm(-3) grown on c- and r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using Cd powder and H2O as source materials. The n dependence of the optical gap energy (E-opt) could be explained by the combination of the band gap widening due to Burstein-Moss shift and the band gap shrinkages due to the electron-electron and electron-impurity interactions. For all the films, the carrier concentrations (n) were independent of measurement temperature (T), indicating that these films were n-type degenerate semiconductors. The barrier heights at grain boundaries determined from the 1000/T-ln(mu T) curves were smaller than the thermal energy at 300 K, suggesting that the grain boundary scattering plays a minor role on the carrier transport in comparison with the intra-grain scattering. The n dependence of the gradient of the mu-T curve revealed the continuous transformation of the dominant intra-grain scattering mechanism from the phonon scattering to the ionized impurity scattering with increasing n. (C) 2014 Elsevier B.V. All rights reserved.