The shift in the band-gap experienced by CdO is treated theoretically as approximately the chemical potential minus the optical potential. These potentials are assumed to be dependent on the partial pressure of oxygen in the cadmium deposition process since the shift depends on this pressure. We obtain an approximate relationship for the electron-hole scattering length from that (which is established here as main result) the chemical potential is roughly null when the CdO electron concentration is maximum, which (as we will show) is equivalent to say that the chemical potential becomes approximately zero if the optical transmittance in CdO is maximal. (C) 2015 Elsevier B.V. All rights reserved.