© 2016 Author(s).Alloying amorphous GeO2 with Y2O3 or related group IIIA oxides is known experimentally to improve its properties as a gate dielectric in field effect transistors. The mechanism of this is studied here by density functional calculations. The metal site coordination is found to be 6-7, by increasing the oxygen coordination to 3 or higher. The alloying is found to increase the bulk modulus. Alloying also increases the diffusion energy of the oxygen vacancies in GeO2 next to the metal and also increases the vacancy formation energy of oxygens that are second neighbors of the metal sites. In this way, a relatively small metal concentration can reduce the O vacancy diffusion rate and thereby the GeO evolution rate. Oxygen vacancies at the Ge/GeO2 interface next to a metal site are found to divide into two types, those which rebond across the vacancy (La, Hf) and those without rebonding (Y, Sc, Al), the latter being preferable as they do not give rise to interfacial gap states.