Molybdenum doped indium oxide (IMO) thin films prepared by aerosol assisted chemical vapour deposition (AACVD) show significantly improved charge carrier mobilities as compared to nominally undoped films prepared by the same technique. The basis for this very unusual behaviour has been investigated by density functional theory calculations using a hybrid Hamiltonian, mobility modelling, X-ray photoemission and X-ray absorption spectroscopies. In contrast to previous claims that Mo acts as a three-electron donor, it is shown that substitutional Mo traps two electrons in localised states falling within the bulk bandgap and thus Mo is a simple one-electron donor. At the same time there is very little hybridisation of Mo 4d states with In 5s states at the bottom of the conduction band. This results in conduction that is spatially separated to some degree from the donors, giving rise to significantly reduced ionized impurity scattering, enhancing the carrier mobility. This is in contrast to Sn-doped In2O3 where the conduction band minimum has significant Sn 5s character, resulting in regular ionized impurity scattering.